I have three in a 10 right now and it's working, but only because it was 5 females in a balanced sorority and we had an illness come through. These three are used to each other. I am, however, trying to increase the fish in the tank, but am looking at non-betta tankmates. These three are actually peaceful together and I don't want to disrupt that with more female bettas just now.
We did have only 2 in a tank at one point and it was a complete bloodbath. I wouldn't recommend it.
Lolly the Bully (banished from the sorority), Vacuum, Finnie, Princess Buttercup, Tiger, and Boomer (all girls); Captain America (our token male)
Dog. Children, and Husband
Frank, Sunshine, Peanut, and Mr. Muscles