Rush ranks their waitlist around May15th. I think there is about 150 on it at its "fullest." I say fullest b/c people will most likely drop out of it as they get pulled off of other waitlists/accept offers from other schools.
From the ranked list, past years have shown that people in the top 25% usually end up getting in, while just about everyone in the lowest 25% has to start making other plans.
Keep in mind that this is not exact, and every year is something new. So who knows, maybe Rush will really go deep into their waitlist, or maybe will only pull out 2 or 3 people? Only time will tell.
Gook luck!