locked out of seller central after changing my phone number

Once you're back in, add an authenticator app straight away as well as the new number. Two minutes, and a phone change never locks you out again.

Did you ever set up an authenticator app or a backup number for two-step verification? If so, choose a different method on the code screen and you're in. If not, on the screen asking for the code there's an option for when you can't receive it. That starts Amazon's recovery, which asks for an ID that matches the account holder and usually takes a few days. Annoying, but it works. Don't open a second seller account in the meantime, that can get both of them in trouble.