Oh boy! That's great news!
I'm so excited!
But down to numbers..I was thinking nine or eleven?
Where do I even get that many?!
And quarantining in cups in the tank is a good idea, but how do you stop possibly contaminated water from getting in the main tank?
Obviously I wouldent use the cups they come in, but for how long could they stay that way? With daily w/cs?
No, not cups in the main tank. Just cups, preferably under a light so there would be some heat. Rotate them around so each girl gets to see other girls. Make notes on who flares at whom, and if one or more of the girls flares at everything and everyone. She/they are likely to be the more dominant fish, and would need to be added last, after the others have a chance to check out hiding spots.
You can buy bettas on aquabid, of course. But most of the time, my Petco and Petsmart have a half dozen or more females. If you only saw four (just an example) you liked at one store, you could get them, line 'em up, and see what you can find elsewhere. Quickly, to get them all in your quarantine process at roughly the same time, because they all have to go into the main tank on the same day.
These animals live (note I did not say thrive) in cups for quite a while sometimes in the stores with daily water changes.
In a true sorority, the main tank IS the quarantine tank, because there are no other fish.
As to the numbers, I'd use AqAdvisor
to get a rough estimate of how many before you had bioload issues. Bettas are not terribly tough on the bioload, but also keep in mind your other fish, and possible aggression toward them with an army of females.
So just because you could have 11, doesn't mean it might not be a bad idea, when given all occupants of the tank.
ETA: Also remember, the more females you add, the more back-up plans you have to have. I suspect little breeder boxes suctioned all over your main tank isn't quite the look you're going for...