Its no less than 4 together, however 6 is prefered. Also I believe its customary to have more than a 10gal for a sorority (I would say 20). I just set up my first sorority not but a few days ago after tons of reading here. Not to promote my own thread but you can read its scaryness HERE
Keep in mind they need lots of hiding spots (lots of live plants and huts/pots are preffered) and to have more than enough room to get away from each other when needed... I just don't feel a 10gal would give you that much freedom.
It will most likely take a few weeks to get all the girls gathered and used to each other (Never settle, wait until the girls you are looking for are available even if it means buying one a week for 6 weeks). Also during that time you need to cycle your new tank.
Plants that I like are Swords, Wisteria, Anubis. The 2 former the betta like aswell.