I only have one apple device and it is sending code to old phone number
only one applesDevice and Apple ID is doing two step authentication and is sending code to old phone number
iPhone 8, iOS 13
Question marked as
2 Step Verification or 2 Factor Authentication
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