Thank you for your question.
Re: "What do you do if it’s a spam call with the 20 numbers attached to it"
You can: Block a FaceTime call and report it as spam on iPhone
Am no expert ... seems it's rather like phone calls :
We don't have to accept calls from anyone we don't know, or we can just leave when we want.
When someone invites you to join a Group FaceTime call, you receive a notification of the incoming call—you can either join or decline. See Answer a FaceTime call.
We also have the option to block a caller, or set up restrictions in ScreenTime:
eg: We can allow only calls from people in our "contacts" if we want.
See how to: Block calls and messages with Screen Time on iPhone