Maximum number of accounts on phone?

"The maximum number of free accounts have been activated on this iPhone." -->

'If you see a Could Not Create Apple ID or Could Not Set Up iCloud message', click on this link --> https://support.apple.com/HT210743

This message generally "means that you have exceeded the number of new Apple IDs you can set up with iCloud on a single device in one year." See the recommendation in the link on alternative methods for setting up iCloud accounts, e.g. on a computer.

The limit only applies to creating new accounts. If you have an existing account you can sign into it. There is no limit on signing in to an existing account.