how many bids does each sorority usually give?

On average around 90 per house.
There will be closer to 2000-2500 plus that go thru rush .
Most people who complete the process get a bid somewhere but may not accept. Others drop after a few rounds because they get cut or it’s not for them.

There is not a set number of available bids at the beginning of recruitment. Cuts are based on return rates, not to eliminate pnm's to get to a certain number at the end, so a larger number of pnm's rushing does not mean more competition; it will mean larger new members classes. Quota is not set until the pref round. If you attend the pref round you are on the bid list somewhere for those houses (or that house if you only have one pref.) If you rank where you pref you will get a bid, guaranteed.