This phone number is already associated with another Xfinity account.

This means that since I used my cellular number on one of my secondary accounts first I have no way to add a recovery number to my primary account.

What am I supposed to do if I need to recover my primary account?

That is a great question, @Bellingham-Bob! If you only have one mobile number, it should be associated with your primary profile instead of a secondary. You should be able to log in to the current primary profile and transfer primary rights to the profile with the mobile number. You can also update the email addresses from the primary profile, if you wanted to have a specific email associated with your primary profile. You can find more details about how to manage everything here: https://www.xfinity.com/support/articles/primary-and-secondary-accounts