Here are the numbers I found on Wikipedia for the UAL/CAL combined RJ feed.

66 50's skw
12 50's chit
17 50's trans
234 50's + 6 135's exjet

175 70's
16 76's (Q400)
336 50's
720 Total

The way I understand, the company can get 64 more 70/76 without consequence. At the 65th 70/76 seater, the 450 hard cap kicks in and they have to park 142 50 seaters. That would leave Expressjet with a 92 aircraft fleet. Or they can decimate the 50's everywhere else and that leaves Expressjet with 187 50's.