how many girls during prefs at each house

It is a different number for each house. The computer system analyzes the return rates (% of girls with invitations who also ranked the house as a 1) and ensures that each chapter invites the right number of PNM's back to ensure they'll make quota on bid day. Strong recruiting houses invite fewer girls back.

Regardless, you don't need to worry about the number of girls at each house because quota is the total number of PNM's attending 1+ pref party divided by the number of houses on campus. If you list all of your pref houses on your Pref card, you WILL bid match to one of them.