You could put a lot of girls in a 20g to be honest, maybe even up to 14 or more.
Since you'll have live plants and I'm assuming the tank will be cycled you could have a lot of girls in that tank, maximum of maybe 20+? Not sure, I have never had a sorority but if you're just gonna do female Bettas you could have a lot of them.
But the higher the number the higher the risk, you could get a lot of girls that just play nice in general and you might have to return them/or place them in their own tanks.
personally i think the more the merrier in sorority situations. i'd do minimum ten, maximum probably 20- if there are a TON of plants and hiding spots, proper filtration, and like Micho said, the tank is properly cycled and you don't mess it up.
Yep, you can fit 20 girls in a 20 if you have no other snails/inverts/fish but ideally 16-18 is a good range to give a little room for moving for you and your girls ^_^ and of course lots of plants, live or real up to you, and hiding spaces for them ^_^
Actually, the higher the number the lower the risk. This is because with more girls the aggression is spread out. However, the risk of losing more to disease rises.
The general rule with sororities is that with proper filtration, water changes, and setup you can have one girl per gallon, so you could fit 20 girls in. That being said, I wouldn't go with that many... Anything over five (six is better) will work.
Like MattsBettas said, you can go up to 20 in a 20 gallon provided you have adequate cover and your filter is able to handle the bioload of those females.
Many people are conservative with how they stock their sororities, but the more females you have the more the aggression is dispersed rather than focussed on individual fish. It's the same theory I find many cichlid keepers work on.
However, in a sorority the importance of quarantine cannot be stressed enough. Even if all your females come from the same source I advise a minimum quarantine period of two weeks. Sororities (even the best of them) are a stressful environment and if you put a female that hasn't been quarantined into your sorority you are taking a huge risk. Many times, if you look at cases where sororities have fallen apart due to disease it is because of improper quarantine procedure.