Zebrasoma scopas: 3.
Zebrasoma flavescens: 2.
My specimens chased and threatened each other a bit during the first week(s), especially in the evenings (over sleeping places, it seemed). Adding all specimens at the same time should help reduce aggression, otherwise try placing a new arrival in a cage for a few weeks until the others get used to it.
My tank is a 55gal, with lots of swimming space and just a little live rock. The fish were about 3" long when I bought them (juveniles) and died of ich the same year, so I can't say how they'd behave long term.