I'm coming from the perspective of an NPC sorority alumna. KSUViolet06, ASTAlumna06, and WhiteRose1912 are also all alumnae of NPC sororities. (Edit: DeltaBetaBaby is also an NPC alumna - she and I cross-posted.)
NPC sorority membership selection is private. No chapter will ever tell a PNM why she was released. That said, every NPC sorority has a minimum GPA requirement, so, if a PNM's GPA is low and she is released, she can make an educated guess that her GPA is the reason. So, you may want to give out letters of regret, but you are under no obligation to explain why you didn't offer a bid to a particular woman.
Since there are only two sororities on campus and both are local, you are under no obligation to cap your new member class size. That said, with 50 actives, you probably don't want to take a NM class of, oh, 200 ... you'd be overwhelmed.
It's a good idea to steer away from alcohol, and I'm glad that you're doing so. You want to be sober while choosing new members. My chapter has held mocktail parties - there are plenty of creative drinks you can prepare without adding booze. (AEPhi has a no-alcohol policy for official functions including recruitment, and the drinking age down here is 21

).