I would say either 3 or 5. Having only two simply doesn't give enough power. Although I could understand doing it for PvE if you were going tank and healer. But two DPS isn't powerful enough to kill anything and stay alive.

The good thing about three is that you can still party with two friends.

Four is fine, but some say that you might as well go 5. However, most of the high rated multiboxers are 4-boxers because they let another person be the healer.