how often do girls drop after pref?

How?

If you maximized your options after pref you’re eligible to a “quota addition”. If you only rank one chapter when you have 2 to rank you are not eligible to be a QA.
Basically they set quota after pref round based on how many PNMs are left and some math based on previous years. Let’s say this years it’s 50 (it’s gonna be more than 50). So after pref the sorority puts together its ideal list of 50 girls for their pledge class. And anyone after that 50 would be great additions too. You don’t get invited to a pref round if they can’t see you being a member of their chapter.
So let’s say 10 girls from their ideal 50 go elsewhere because they rank their other groups higher. That’s fine they move down to girls 51-60. Then they have a full pc life’s great. If girls 61 and on didn’t get bids to their other chapters and maximized their options, they’ll be added to the PC as quota additions.
That’s a super basic way of explaining it there is more detail than that but I’m exhausted so that’s it for now...

Don’t forget you can be a QA to your second choice too....maximizing your options doesn’t garuntwe you your first choice but it helps a lot more than suiciding (only ranking one chapter if you have 2 left). And no one knows you’re a QA even you you’re just an initial part of the pledge class.
Today something else known as “snap bids” will be given out once bids are finalized but before the bid opening stuff. That’s when a sorority is short new members for quota so panhel and that sorority will call girls who DROPPED OUT OF RUSH on their own but who were STILL BEING INVITED BACK BY THAT CHAPTER HAD THEY NOT DROPPED and say hey we know you dropped out but we thought you were cool would you like a bid and then the snap bidder girl goes to bid day and no one but her and her chapter would know she’s a snap big unless she starts telling people. Don’t bank on a snap bid and most “top tier” and “mid tier” chapters do not need to do them because their pledge classes are usually full. Based on the return rates this year it’s probabblyDG and tridelta who will be snapping MAYBE theta.