There is a limit to how many times you can use the same phone number. I don't know the limit number. It is very unlikely that personal accounts will ever encounter that issue but if you manage multiple apple id's for clients then it eventually will come up.
3 solutions the way I see it:
- if you use a landline (that can receive texts) as the trusted phone number see with your admin if you can have aliases.
- Delete old, unused apple id accounts (hard to do if the customer decided to use personal account or has changed the password)
- Enroll for the Apple Business Manager (free but may require MDM)