Do I need a SIM for iWatch 6 cellular

The SIM is built into the watch.

M1 Mac mini, macOS 11.2.1; Watch, series 5 GPS/cellular, watchOS 7.3.1; iPad Pro 2nd Gen, iPadOS 14.4; iPad Air 2, iPadOS 14.4; iPhone 12 Pro, iOS 14.4; TV 4th Gen, tvOS 14.;  HomePods, audioOS 14.4 and Airport Express w/ Airplay 2

The Cellular + GPS 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

In the US the majority of providers charge $10 a month for the Watch plan, but T-Mobile/Sprint charges $15 a month with a $5 discount if you use autopay* for your phone bill.

I have T-Mobile as my carrier and yes, my iPhone's calls ring on my watch, but my watch has its own telephone number.

*Autopay is where you permanently supply a credit card on your cellular account and the carrier charges your bill to that card each month.

BTW, if you pay your cellular phone bill with a Wells Fargo credit card (not a debit card), your cell phone is covered with insurance against damages, theft or loss for free.