I would think 4 or 5 females would be fine in a tank that size. I know a breeder that has about 20 in a 29 gallon. They squabble, but once the pecking order is set, they tend to tolerate each other.
I would not, say... put 3 females in a 3 or 5 gallon tank. One would be dominant, the others would be forced to hide all the time.
The thing to remember will bettas, they are naturally territorial and solitary, only together for breeding. So they really don't like other fish in their space.