I'd buy 80 females and 10-15 males if you can. They give you an 20% overcount everytime so you get even more. Most of the females are already pregnant so you'll start to see babies quickly and after about two months they should be about feeding size.
If you just want to get some to feed off, buy small or mediums. Or you can buy enough for a colony AND get some for feeding.
My colony has about 70-80 females and a good 15 males with hundreds of nymphs right now.