I think you would need more than 300 rounds even if you are just walking around looting, you never know there might be a pack of bandits headed your way. And also to add the fact that you could barely find friendly people (idk, based on my experience)..
Exactly what I have in mind right now lol, but i'm guessing that the reason why they don't put too many zombies is that the increase of zombies is proportional to the increase of lag people would get, i don't know, correct me if i'm mistaken.