do I need a new sim card in my series 7 watch

The Apple Watch shares the iPhone's number. The carriers that support the Apple Watch eSIM charge a fee on the iPhone's monthly bill. In the U.S. it is typically $10 U.S. per month. Here is a list of carriers that support the Apple Watch

https://www.apple.com/watch/cellular/

If you have a GPS only Apple Watch, it will do everything the cellular model will do As Long As the iPhone is nearby.

https://support.apple.com/en-us/HT205547

There is no physical sim card in a cellular Apple Watch. It uses eSim technology.