You can capture crowd during a concert on several ways. First of all, it depends quite a lot if you want to record the audience of a classical or rock event. if it's loud as hell on a rockshow, you will get the pa loud as hell on the microphones.
-Classic shows: if you use overhead microphone to capture an orchestra, normaly the level of the audience is quite nice on it.
-Delay/amby line: hang 2 microphones directly over the audience, away from the stage
-if there is a live-record, place a stereo-set at the FOH-place, one in the front, one goes in back. record as much as possible and check what you can use
-Openair-Festivals: If there is no ceiling to put on overhead mics, there is a big stage with lot's of truss. hang some amby mics, directed to the audience on the front of the truss: a stereoset in the center of the stage, one microphon left, one right. Also place microfones at FOH place.
All this requires preparation, takes a lot of time (except placing 2 miks at FOH, this is easy), gear, long cables, channels for recording and good microphones (schoeps, neumann).
The fact is, you need something placed on the audience. If it reacts, you will have something. If it's a good show, the audience is very loud, no problem! But if you compare for example with the Rolling Stones DVD, they record every show, cut together the best things. And also, the FOH place of such a big show is far away from the stage, and the direct sound from the pa is much smaller than the level in a club or small venue.
I have some audience reaction on: https://www.soundeffects.ch/surround-sound-effects-_e.php?soundeffects=concert+crowd&soundcategory=&type=
sometimes, boxing-ambience or sport ambience can help, but most of the time, it does not work.
I'd try to record the audience, if the pa is too loud, try some tricks, filtering, to phase-inverse the LR-Output from the mixing-desk (which should be recorded too!) to the amby-channels and so on. If it's too loud, than it's too loud, the only thing is to run a show on quiter level for the record.