Is it OK to use my cell number for iPhone and Apple Watch 6?

The 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

It is the same number.

The number your provider gives is only for their invoicing purposes.

Everyone will use your main number and it will come in on both your iPhone and watch.