In my personal experience:
I kept 2 betta females in a 5 gallon tank. They were both originally living on their own in smaller tanks, and BOTH their filters died on the same day, so I thought I would give it a try. I didn't know they needed plants, so they had a couple small caves and a couple of short plants. They got along incredibly well - they became best friends. Then one got sick, and never recovered and died. The other one died not a couple days later - she was healthy, but I guess she was lonely without her new best friend.
So, my experience, 3-5 females would do fine in a 5 gallon. My two were best buddies - no reason why they wouldn't make friends, imo.