I have 10 Female Bettas in my 30 Gallon tank.
The Sorority started out in my 10 Gallon tank with about 4 or 5 Female bettas. Then I bought more Females to add to the 10 Gallon (We all know how hard it is to resist Bettas in the store looking all pretty and sooo tempting to bring home! LOL) I was pushing the limit. So soon after, I bought a 30 Gallon tank that came with the tank stand, hood, light, some decorations, heaters, air pumps, fake plants, brine shrimp eggs, test kit, plus more at a garage sale all for 40 dollars!
I agree that 10 is a good size to start a sorority. Just don't push the limit of fish in there like I did.
(Hard to do I know because the Bettas almost beg us to buy them and bring them home)