Which iphone do you need in order to open an apple savings account?

Any iPhone that can run iOS 16.4.1. See→Set up and use Savings in Apple Wallet - Apple Support

Note all the requirements:

• Be an owner or co-owner of an active Apple Card account and add Apple Card to your iPhone.
• Be at least 18 years or older.
• Have a social security number or individual taxpayer identification number.
• Be a U.S. resident with a valid, physical U.S. address.
• Set up two-factor authentication for your Apple ID and update to the latest version of iOS.

Any iPhone using iOS 16.4.1 or later will work.

And, of course, you must meet the requirements stated in the article linked above.