The problem with a situation like that is you're going to have a tough time proving that deadly force was the only option. A gun would be a deterrent, but something like pepper spray would work far better in that situation. Even better would be putting the car in reverse and getting the hell out of there.
Guy sitting in car gets punched... not deadly force. You couldn't really argue deadly force until it got to the head kicking/stomping.
Of course, if you brought a gun out before that, you could argue deadly force if he tried to disarm you. But with 20 of them there, its probably not worth it as one of those pieces of **** probably has his own gun to shoot you gangsta style in the back.
I keep a can of pepper spray clipped to the underside of my car seat for those kinds of situations where its not to deadly force yet. The gun never has to come out if that works.