Also, asides from adding them at the same time, you'll also want to re-arrange all of the tank decorations so it's like a whole new tank for the older female as well.
oddly enough i did that a couple days ago
would it be too hard on my current betta to add more than 2 at once?
Well, if the tank is the right size to hold more, here is what I would do. (A couple of days ago won't really help, as your already owned female will have already gotten used to it again)
Are you ordering the females from online or getting them from a store? What I would do is take a spare cup/piece of tupperware or something else you would feel comfortable putting your female in temporarily. Get your new females, but dont add them to the tank right away, instead, pull out the original girl and place in the temporary container with water from her tank, then while all of them are out, re-arrange and or add or remove pieces as you see fit, then put all three or four or however many you get in the tank (Still in their cups) so they can see eachother bot not attack, do the acclimating for the new fish (you know, the whole, take some water out. Add some tank water. Repeat multiple times stuff) Then, once the new fish are climatized to the new water, I usually just tilt over all of the cups and let all the girls loose at once.
You'll have some flaring, some chasing, maybe a little bit of nipping for the first 3-5 hours or so, depending on the amount of fish and their personalities, my girls settled into their hierarchy after about 2 hours... so you'll want to watch them closely during this stage.
