Graphene has nothing to do with your sim or phone number, your sim card is possibly tied to your IMEI but that never changes so it should be just fine. If for some reason it won’t work after you flash graphene your phone provider should be able to swap you to a new sim card and port your number over no problem, but even that would surprise me, it should “just work.”
Btw Whatsapp and Gpay can run on graphene but it sort of defeats the entire purpose. Some people will create an alternate user profile for all that type of shit and install google play services on it, and put all their spyware apps over there. Just something to think about, it’s easy to set up once you have graphene installed.