I have 2 watches in my ATT account, how do I know which phone number is mine?

The number on your watch is the same as the iPhone it is paired to.

While the watch may get a number, it is for billing purposes only and cannot be called.

Any Watch on your plan should be paired to your iPhone and as such take on the iPhone’s own number.

If you use Apple Watch Family Setup, then the number would be whatever it was setup with, there’s no place to see that number other then on the monthly statement for the watch service plan.

I don't believe you can see your phone number on your Watch. The number you see with AT&T is just an accounting number used for billing purposes and not the actual number you call. People would just call your iPhone number to get the Watch to ring.

This doesn't work with all carriers which is why Apple does not display the number.