I forget two factor authentication Number

You need to change/add the number on your Apple ID. After the change, allow up to 24 hours for the change to sync. Before trying to make the change, sign out of Messages/FaceTime on all devices/computers.

Apple ID - Manage      

See if anything here helps.  If necessary, you can use account recovery as explained in the link at bottom of the article.

Verification code - how to get one and sign in with two-factor authentication.   

You forgot your phone number?

2 factor authentication will send a new code to your trusted device(s) whenever you try to log in from a new device.

It changes every time.