What is the optimum library size?

It will really depend on your patience, how large you will let the library grow, also, whether you are planning to use iCloud Photos or not. If you are using iCloud Photos the available storage on your smallest device will be the bottle neck.

I asked at the Genius Bar at the Apple Store in Hamburg and in Berlin, and both told me not to let the library grow beyond 20000 photos, but that will only apply, if you are using iCloud Photos and have to consider to keep your library small enough to fit on your iPhone with the smallest storage, also on your patience, if you cannot stand to have to wait a few seconds, before a photo opens.

Photos is supposed to be able to hold a million photos, as Yer_Man pointed out, but some tasks are annoyingly slow, when the library is getting large. For example, upgrading the library to a new system version can take more than a day, if the library is large. Also, the artificial intelligence scan for faces, objects, screen recognition and the advanced search is needing a lot of computation time and will be slow, if the library is large.

My System Photos Library is just having a moderate size of 80000 items, but after the upgrade to mac26 it took more than week to scan the library again, before the search found anything.

Be prepared to wat for day, if a large library needs repairing.

It will also depend on the number of albums and smart albums and keywords you are planning to use. The graphical user interface has not been designed for a large number of albums and keywords. It helps to keep the Photos Libraries focused on one thematic project, so we do not need many albums, and keywords.

My System Photos Library is heavily structured with folders, albums, smart albums, and on my Mac in Photos I can find all photos quickly, but not in the Media Browser, it is only showing a flat list of 2000 albums, and not on my iPhone and iPad, because the smart albums and keywords are missing.

Also the list of keywords is hard to manage, because the Keywords Manager is also only showing a flat list of keywords.

I have to keep the number of smart albums small, at 50000 photos and 200 smart albums Photos started to hang or become very unresponsive. Now I am deleting most smart albums as soon as I am done with the corresponding photos and copy the photos from the smart album to a standard album. When I start working again with the same albums I recreate the smart albums for that project.

". I've scanned thousands of film negatives, slides, and prints, and consolidated thousands more digital photos sourced from everything from a Palm PDA to iPhone 16 Pro. "

That is what I have done too. My library is including scans of the old family photos from the time of glass plates, including scans of old family documents and newspaper cuttings, etc. It keeps growing. It started with only 20000 photos, but then I added also the old family videos, and after I purchased the first iPhone and had always a camera with me the library kept growing faster and faster.

So I suggest to plan for a growing library on a volume with plenty of free storage. Photos needs a lot of free storage when it is rebuilding the library after a system upgrade.

And better be safe than sorry. Richard has a very valid point about the backup. In addition to your regular TimeMachine backup keep a copy of the Photos Library off-site. I am keeping a copy of the backup at my place of work. And are using iCloud Photos to have my system Photos Library stored on Apple's servers as well.

When I want to watch my photos on Apple TV, I I am creating shared photo albums. They are easily accessible on TV, and we can have up to 200 different shared albums. we do not even have to use iCloud Photos for this. The storage is free.

The shared iCloud Photos Library is available on a Mac with macOS Ventura or later, or mobile devices with iOS16.1 or iPadOS 16.1 or later, but not yet suitable to share albums, I am sorry to say.

See: How to use iCloud Shared Photo Library on your Mac - Apple Support

When you share photos to a shared iCloud Photos Library, they arrive in the library of the subscribers as a flat, unorganized stream of photos. The subscribers have to recreate the albums.

Richard has pointed out another risk and I can confirm it: Anyone who is taking part in the Shared iCloud Library can move photos to the thrash, change the adjustments, the titles, everything. When I started to use the Shared iCloud Photos Library I shared only duplicates of my photos, and pretty soon stopped sharing with others. Sharing a copy of the library with others on a portable drive, so they can copy the photos they want to their own devices is much safer. But this requires that all Macs are having the same system version installed.

The Shared iCloud Photos Library is currently the only way to share a Photos Library among devices with different system versions installed. For this purpose, the Shared iCloud Photos Library is a great new feature. But we have to use our own System Photos Library to take part in a Shared iCloud Photos Library, because only our iCloud Photos Library, our System Photos Library can be enabled as a Shared iCloud Photos Library. We have to share from our most important library, and allow others to modify the picture. That requires a lot of trust in the technical skills of the people we are sharing with and all of them have to be considerate. The photos we share or are shared by others will show up on all our devices where we are using iCloud Photos, also on our devices with limited storage.

A lot depends on the Mac you're using, i.e. amount of RAM, amount of free space on your drive, where the library is located, i.e. boot drive or external drive, how the EHD is formatted, if an HDD or SSD, etc,, etc.

I have a 55,000 photo, 900 video library on an external SSD with a read/write speed of 500 MBs (which is slow compared to the more expensive external SSD/s, that opens in 2 seconds. This is the budget setup I use:

A more elegant solution and one easier to carry around with an MBA is this OWC Envoy Pro MINI in sizes from 500MB to 2 TB:

If you have a silicon Mac, a minimum of 16 GB of RAM and plenty of space to hold the library (mine is 187 MB) a 70-80 thousand library would be doable. The plus of having all of your photos in one library is when you go to group a specific event or person in an album or Smart Album they are all conveniently located in one library and can be easily routed without having to search multiple libraries and exporting and importing photos from one to the other. NOTE: this is without using iCloud Library!

Just a quick note: the library cannot be on a NAS it must be on a disk formatted Mac OS X Extended, Journaled or apfs and directly attached to the Mac and not a networked location.

If this was for professional use I would not be using a giveaway app like Photos.

Theoretically, there is no limit, but the optimum size is dependent on things like how much disk space you have for storage, what Mac you have, how much Ram you have and so on. Other factors: typically a 5mb jpeg will open faster than a 40mb raw of a 100mb tiff. SO what you're managing with your Mac is a factor too. SO the short answer is... trial and error. There is no one optimum size.

If you are talking about a very large number of rod images, then Mylio is very good at that, while also being very user friendly. At least one use of that app I know has more than 4 million images managed in it.

I like the idea of single family Library, but it made me think: What happens when someone else deletes all your pictures? I mean accidentally, of course! That will be copied everywhere, on all machines, all at once. You can't depend on iCloud alone. 

It sounds like you need one Library to gather and curate your pictures, editing them to look their best, and choosing the ones you want to look actually look at are often. Then you export those, perhaps with a smaller size and useful filenames, to a flash drive, and plug that into the TV or digital picture frame or whatever. You could put that on iCloud Drive, and everyone could access that.

Depending on exactly what you want to achieve there are ways and the most simple way is to put the images online, and that way they can be accessed by anyone with a computer/phone/tablet or even a TV. But now you're into cost, and that's inescapable.

The most basic way is to create a website yourself. Register a domain name and buy some webspace from a company. Then create a website with a password - not difficult, and you can create a good looking site quite quickly with free templates from Wordpress or similar.

Use a Photo sharing site like Flickr or Smugmug. You buy a membership and can upload all your photos and possibly videos too. Pay the annual sub and they'll keep them up there forever. Again, you have settings that allow you to limit access in any way you like, to insure privacy, and anyone with a web browser can get to see the images.

Adobe Lightroom - getting more expensive but possibly the best option. It's an app like Photos (with more bells and whistles) and you import all your images to it, and it stores them online and from there you can create and share albums (again, privacy protected) and effortlessly create websites with it. I use this for more than 100k images. If you want to edit/improve/ the images this is your best option also. I'm the only one in my family who has the app, I share the rest via web browsers.

Each of these options is platform neutral. Folks looking at the images would not need to be on a Mac, or using Photos, these will be accessible to anyone with a web browser.

There are other options but those are your basic ones, and the others are variations on that theme. You wouldn't even need to stop using Photos, especially in the case of the first two. And, of course, while all your images are online, you would need to maintain a back up locally. That's just common sense.

léonie wrote: …resolution reduced to 2048 pixel at the longest edge.

I think that's OK for shared pictures, since you don't edit or print them. But

Our titles and descriptions will not be shared, we have to copy them to a separate comment.

that is really nasty. No faceIDs, no list of people in the caption or title, and no keywords. Drat!

Shared Albums seem like a neat way to distribute pictures since that doesn't count against the Cloud Storage. And NewNurse57 wants every family member to be able to add to the collection, which leads to the danger of accidents--but Shared Albums are separate from your own Library, so accidents won't be as potentially costly. Apple says:

If you're the owner of a shared album, you can delete any photos, videos, or comments from anyone. Subscribers can delete anything that they personally added. Anything you delete is automatically deleted from the album on all your devices and the devices of subscribers. [But they are only deleted from the Shared Library, not from the owners own Library.]

Each family member would be in charge of their own pictures-- they wouldn't be added automatically. If I were the "owner" of the Family Library, which must be the System Library on a Mac, I would want my own main Library to be separate-- but then it couldn't be the System Library. I think that would work.

It's also possible to access Shared Albums with a link, I think. (Frankly, I haven't done much with Shared Albums, so you can't really believe anything I say. I don't.)

So what's the down side?

The downside of Shared Albums is, that Photos is optimizing the shared items for sharing - metadata will be stripped for privacy reasons and the resolution reduced to 2048 pixel at the longest edge. It makes it hard to share old family scans, when we want to describe the people in the photo. Our titles and descriptions will not be shared, we have to copy them to a separate comment. And the comments will not be saved, when we download the photo from the shared album. Depending on the system version, the dates and locations we added will also be missing.