I had 3 - well, 4 I guess...I had a MOH and 2 bridesmaids, then DH's daughter was a Jr. Bridesmaid. He had 2 best men so one of the best men escorted the Jr. Bridesmaid. I would have preferred not having the whole "jr" thing, but guess I couldn't not do it...she's his kid and all...

I didn't pick based on any set formula - I have 1 best friend and 2 women who I'm still close with since elementary school so it was obviously them.
Quite frankly, I couldn't imagine having more than that. I still got drama with only 3 (ok, it was only from one of them) so I'd never want to increase that!
In terms of paying, I think most women when they agree to being a BM think they will have to pay for the dress, accessories and alterations. I personally think if a bride makes all of them get their hair and make-up done the same, she
shouldpay for that. I also think it's ridiculous to expect everyone to get Vera Wang bridesmaid dresses and Jimmy Choo shoes.
I personally paid for a little more for one of bridesmaids b/c I knew her financial situation and I could afford it. For the one who flew in with her husband, I paid the hotel for the night of the wedding. And for my MOH I paid for her hair as an extra gift for being such a great friend. It's all really up to you are what your budget dictates.