Phone and watch need different numbers?
Your iPhone (12) and Apple Watch (6) need each their own phone number to use the watch in cellular mode?
Question marked as
Top-ranking reply
The Cellular + GPS Apple Watch shares the iPhone's phone number, and the carriers that support the Apple Watch charge a monthly fee. In the U.S. the fee is $10 per month. Here is a list of carriers that support the Apple Watch:
https://www.apple.com/watch/cellular