Hi there!
I currently have a 20 gallon tank that houses a sorority of 9 bettas.
In my opinion, odd numbers are best because in some cases, you will find that two of the girls with fight for complete dominance and it could lead to the death of the other, throwing off the heirarchy. I would recommend at least 5-7girls for starting a sorority. It is best to add them all at once but I have had no problem with adding new members so far, it depends on your fish.
Never keep males with females. Even if there isn’t an immediate problem, the male can quickly turn his attitude around. Even when breeding bettas, you should remove the female once the male is “finished” with her so no further trauma comes to her.
Clean water, good filtration, and proper space are important in keeping sororities, but some people support the idea that a slight crowding of the tank actually helps distribute any future aggression so that it is not focused on just one fish.
If you are going to do a sorority, make sure there are enough places for the ladies to escape from each other, females are just as tough as males, just slightly more tolerant. Also be sure to inspect fish before purchase and monitor your tank for any developing diseases or over-aggression. Keep in mind that some females simply will not tolerate others so you may have to return or separate a bully.
One more thing: there is bound to be some nipping and chasing in the beginning, so expect some fraying, but keep an eye out for bullies.
hope this helps and good luck, let us know how you do