Do you have to have an additional cellular line with the series 6 watch?

The Apple Watch shares the iPhone's phone number, and in exchange the iPhone's cellular carrier charges a monthly fee. In the U.S. the fee is $10 per month added to the iPhone's cellular bill.

Here is a list of carriers that support the Apple Watch

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

If you purchase the Cellular + GPS model you would contact your cellular provider and they will charge you to add your watch on your plan. It will share the same phone number.

If you have the GPS alone, there is no add on needed as it cannot use cellular.