If I'm by myself I stick to three. I know that I'm probably only going to shoot two of them but it's nice to get a bit of choice in the matter while I'm there. I find I have between one and two hours of good shooting before my aim goes to pot and that's just not enough time to put serious work into more than three.
On the other hand, if I'm bringing buddies I'll usually have 5 or more. The biggest problem with that is the cleaning afterwards - even now they're glaring at me, demanding attention even though it's been a couple of months since I've been out last.