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It varies based on the return rates of the each specific chapter the last few years. Chapters with a history high return rates for each round are told by PHA to invite back fewer PNMs. Chapters with a lower history will be required to invite back more PNMs. Zeta will be required to invite back nearly everyone since they don't have any numbers.

Chapters can't choose how many PNMs they want to invite back but they can choose how many parties they want to have. Some chapters want to make the parties, particilarly Pref, more intimate so they might have more parties. It might or might not have anything to do with the number they invite back. Not a big deal but some actives on this site think if a chapter has fewer parties it is the only determinant in how many PNMs they invite back.

This is called Release Figures Methodology. PHA at OSU followed the RFM developed by the National Panhellenic Conference.