It all depends on the situation. I voted 5 because that takes care of all 3 holes and both hands. If it's more of a train situation, then the more the merrier, line 'em up!
A gangbang needs many to make it worthwhile, the female need to be manhandled and passed around but she needs to feel wanted by all the bulls so there should be around at least 8 - 10 bulls. This will ensure she achieves orgasm a few times at least.