If you're talking about in general, my rule is one girl per place. If a guy is on the bus, and he hits on one girl on that bus, it's okay. It's okay, because no matter what happens, the world is on his side. Everyone around him knows (both men and women) just how hard it is to start talking to a complete stranger you're interested in. So it doesn't matter if he succeeds or fails; to the rest of the world around him, the guy is a success; the failure is the guy who stays quiet and doesn't do anything.
However, as soon as that same guy hits on a second girl in the same bus, the world's sympathy turns to laughter, spite, and disgust. Now, this guy turns into this desperate dog, begging for attention and a piece of meat.