I voted three because that's how many I have with my male in a community tank and am yet to have any drastic problems, sure the females chase eachother out of where they want to be sometimes and the male chases all three of them out of his corner when he wants to be alone, but that's the closest thing I've had to any real problems. With bettas though you do have to be careful with some as they all have differant personalities and some are more solitary than others, just keep an eye on them for the first few weeks and if they seem to coexist peacefully I.E. not killing or trying to kill eachother, than I don't think you have anything to worry about, exept when/if they breed, which is touched upon elsewhere in this forum.