My slut has had up to 5. All bbc's, it was her bull and 4 friends. They definitely left her stretched and gaping.
While I enjoy seeing Krista dog-piled, it sort of spoils the up-close view. I've seen her do as many as seven at once, but it was less satisfying for my kinks. The seventh guy was mostly a third-wheel so to speak, jerking and getting sucked by the other guys. It also adds lots of jostling around as the guys trade off - kind of a distraction.