how many girls typically attend cob parties?

I got invited to 3; 2 of them I don't really have interest in because I know plenty of girls in them that are catty/dramatic, but I'm gonna go to their parties anyway just to see for myself. As for the one that I really like (my favorite during formal rush, a bunch of girls I am friends with), my friend told me that they don't invite the big list of interested girls, only the ones that their sisters recommend. I'm super nervous, any tips? What's it like? How many girls typically attend these?

Side note: I'm not going to name the sororities because I don't want to contribute to "stirring the pot" or whatever the hell people call it. Just generally interested in info about COB. Thanks!