We have a pasture with 75 cows and I've been running two angus bulls in it.   I've found that more bulls means more fighting.

 I am assuming that you are running the 32 heifers in a separate pasture.  That's what I do and use a bull that is smaller boned and not extremely heavy to avoid breaking down the young gals.

 So, I would put one bull with the heifers and two bulls with the cows.  Test all the bulls and you should be fine, provided nothing gets hurt.