Hello and welcome to thge forum. You can add 2 or 3 at a time. I don't have a sorority myself but the article I wrote was based on research and info I got from other members. If you add them all at the same time then no one gets the chgance to claim territory and then fight with everyone to defend it.
I just read your post about female sororities. I would like to start one, but I don't know a lot about starting new tanks. I would like to start a 10 gal tank. My problem is that I've read when starting a new tank you add one fish at a time, but I will need to add more than one at a time if I'm making a female betta sorority. Any tips?