If I buy an Apple watch Series 6 do I have to have a separte cell phone plan or can i connect to my iphone

You can connect it directly to your iPhone. Even the cellular model does not require a separate cellular plan. It is added to your iPhone’s service plan for a device fee if you choose to activate the cell service on it. But you don’t have to if you don’t want to.

If you never plan on using cell service on the watch, you can buy the cheaper GPS only version.

The GPS/Cellular version of the Watch requires a watch data plan ($10-$15) added to your current cellular plan. So you must have a cellular provider that supports the Watch.

M1 Mac mini, macOS 11.0.1; Watch, series 5 GPS/cellular, watchOS 7.1; iPad Pro 2nd Gen, iPadOS 14.2; iPad Air 2, iPadOS 14.2; iPhone 12 Pro, iOS 14.2.1; TV 4th Gen, tvOS 14.2  HomePods, audioOS 14.1 and Airport Express w/ Airplay 2