You will find many threads on introducing females in this site. Try the search feature, or Google "introducing female bettas" in Google and you will find a series of threads that take you to this site.
Is a good article.
From what I hear, there's safety in numbers. The more females the less opportunity one will single out another to pick on. I will be introducing 6 females in a couple of weeks. I have two right now and when I tried to put them together it wasn't long that they were body slamming one another and aggressively chasing each other. The stress will lead to sickness and death, if they don't actually kill each other. I separated them at the good advice of one of the members of this great forum. They are in separate tanks until the others arrive.
How big is the aquarium you will be placing them? Rule of thumb is 1 female (preferably less) per 1 gallon of water.
Will there be any other tank mates?
Is your aquarium cycled and heavily planted? Your females will need a lot of hiding places as they sort out their pecking order, for there will be a few chases. I have my aquarium heavily planted now, top and bottom (floating plants, dense plants and tall plants), with decor like floating logs and driftwood and decor that the fish can hide in without tearing their fins.
Do you have another small holding place for any female that might be too aggressive? Do you have a plan on what to do with her if she really needs a place of her own?
From what I've read, they all need to be introduced at the same time to the aquarium, so one doesn't have the advantage over the other. They need to be watched carefully and continuously for awhile, for any overagressive female. If there is one, take her out and float her in a clear container in the tank. This way she gets used to her tank mates, but can't harm them. After a day or so, put her back in and watch carefully. If she repeats the behavior, take her out again and do the same. Bettas are smart and she should learn that aggressive behavior only gets her in isolation. There may be some that just don't learn. That's why you need plan B.
I'm sure there is more info I left out. Again, enter the word introduce betta in the Search and see what you get. Or go to the section in this forum for Betta Companions and you will find many that are asking the same questions as you.
Most important, do your research and you will have better chances of happy, healthy girls.