Do you have to have a separate line for the apple watch?

If you purchase an  Watch with cellular and want to use the cellular feature you must add the  Watch to your the cellular plan used by your iPhone. The cellular provider will charge an additional fee and assign a number to your  Watch. That number is only used by the cellular provider. It is not used to call your watch. When someone calls your iPhone's number the call will go to your  Watch also.

No, it uses your iPhone telephone number. You do need to pay your carrier extra to add your Watch to your Plan.