Multiple Remote Users for One Mac at Same Time - Possible?

Technically possible when you make a connection with Screen Sharing it will ask if you wish to share the users session or not. Or login as a different user. If you login as a different user you get your own Desktop environment.

I don't know what the limits are on the number of active logged in users with a remote screen sharing connection. I would guess it's around 10 at the same time. I've only done between 3-5 connections.

The more people you have remotely connecting the more the Mac is going to be stressed. It also matters on the software each user is running, the amount of RAM / CPU, etc. If one user pounds the disk with some file I/O it will impact the other users momentarily until the disk activity calms down. If you can get a Mac with 10GbE Ethernet that may help.

Since it's a client, you could setup a test to see if it will work or not for your client's scenario. Setup the same Mac you intend to recommend. Then have several other Macs make connections via screen sharing. See how hard you can push it till the Mac falls to it's knees due to the heavy workload. Using lower quality video will help. Using a lower resolution monitor will help.

If the user is NOT using Mac clients, then try using the RealVNC client, as it can use the macOS username/password, as opposed to the insecure VNC password and no username. That would be needed to select a different macOS user account.

Also you do not set up profiles, rather you create user accounts via System Settings-> Users & Groups.