How big are the pledge classes for sororities?

There is never a pre-determined number of how many will get released or receive bids. Releases each round are determined for each house by percentages based on the number of PNM's participating, and the size of new member classes is only determined after pref round when all the PNM's have turned in their cards. The basic formula for class size is the number of PNM's who turn in pref cards indicating they will accept a bid, divided by the number of sororities. That is Quota. Then, RFM kicks in to assure that every PNM who ranks both her pref houses, or just the one if that's all she has, is guaranteed a bid even though it would put a house over Quota. That is Quota Plus. So some houses will have larger or smaller new member classes. A PNM is guaranteed a bid if she attends pref round and ranks those houses (or the one) on her card. She is not guaranteed her first choice but she will get a bid. So yes, if there are more PNM's there will be larger classes.