I would personally stop anywhere from 75-125 mobs, just to give other grinder users and darkness spawning a lot of breathing room.
One time I got 400 in there, that is accually too many, just do 200 or so, and than a splash potion of health
Aikar did a thread on it because he's the programer and If you've got a natural spawn grinder he said try not to let it get over 150 because if you have 350 mobs in your grinder no other mobs will spawn anywhere in the nether or wild (with the exception of spawners) if you want to read this Thread it's in EMC guides.