I have to disagree with Jrf. It is much easier to add all the girls you want at once, because every time you want to add a new betta, you have to rearrange the entire tank and cup everybody. It's a pain. Just add all the girls you want at once.
As for cories, at least 6, but you could have more if you wanted. Just make sure they are all the same species (don't add one panda, one bronze, one peppered etc).
It is usually advisable to add the non-bettas first, so the bettas don't see them as invaders. :)