do i keep my phone number when adding cellular to my watch

The Cellular + GPS Apple Watch has an eSIM and 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:

Apple Watch - Carriers - Apple

Yes the watch uses your phone's number,it's called number sharing and you pay an extra fee on top of your existing contract.

This only applies to the cellular version of the watch,a cellular plan cannot be added to a GPS only watch.