If you're planning to breed a bull to the heifers (or cows), one yearling bull can handle 10 to 15 females easily. A 3+ year old bull can easily handle 25 to 35 females.
TWO or more breeding age bulls (12+ months of age) in the same pasture (or across a fence) with females is a large wreck waiting to happen. Two bulls should always be seriously separated and definitely not be able to touch noses...they can easily take down a fence (unless is a 6' high pipe fence...lol).
Cattle will butt heads with EACH OTHER as act of play, establishing dominance. Any animal that butts PEOPLE should receive a sharp rap on the nose with a fiberglas sorting stick (or) if that doesn't cure the problem, then the animal should grow wheels. You don't need a 4 footed animal that is larger (or more powerful) than you to play dominance games with you. People have died from this.
If you had rather have bulls rather than females, then "steer" (castrate) the extra males when they are young...