its just an attempt to spread out the aggression. with just two females the male and even the other female are more likely to focus on the weakest link. with three or more the male is turning every which way and no one female gets too much negative attention. I know I'm probably overstating the aggression. usually its only during and right after feeding where they do alot of chasing, otherwise they dont do much more than occasionally get out of each others way. then at night its funny, all of a sudden they practice safety in numbers. there is no magic number. you may get a pair, three or four to work its just the more you have the more likely they will coexist. one male and three females was the magic number for me (thats all I tried though. went from one female to the 3:1).