Yes, the general rule is 4-6 in a 10 gallon. The reason for this is, if you have less than four female bettas, there are not enough fish to distribute aggression - The fights will be much worse. More than 6 females, you are overtocked (as far as bettas go. The stocking is a little different then with regular fish). You can get away with 3 if they are especially peaceful or the tank is well planted.
10 gallons is generally accepted as the minimum, though an 8 gallon or something isn't going to kill, as long as you keep up with water changes.
If you want to get a bigger tank, then that is fab ^^ I'd guess (complete claptrap, I'm just guessing this - get another opinion) for every 5 gallons you add, you can have 1 more fish. So if you do a 20 gallon instead, you can have 6-8 fish. That's how I'd stock at least
I suggest getting LOTS of plants, I prefer real ones because I think my fish seems to enjoy them more. But many have success with fake plants. If you go fake, make sure you get some soft, broad-leafed plants, so they can really hide themselves. For real plans, my girls love water sprite plans - Thin boad leaves. I put them in my very well lit 20 gallon first and the plants got huge, them moved them to my 10 gallon.
Good luck. I defenitly love my sororety tank
Katy - Posting too much, again