As Nova said, depends on the bull.
One of my big projects this year is a revamped bull pen that I rebuilt on a neighbours property. It consists of three pens covering 6 acres to keep developing yearling and mature bulls separate, as well as a holding/corral area. I looked at building a similar set up on my own place, but trenching for water lines and electric and water bowls, etc...was going to put me in the 8-10 grand range for price. I just completed Phase 1 (Yearling and corral pen) in early June (Cost=~$500). The old fence consisted of 9-10', 6-8" posts that were in the ground 4 feet at 10 foot spacings with 5 barb and one hot wire in parallel. I ripped the entire old fence out as many posts were broken, leaning badly, or ready to break. Reused all 5 strands of barb wire at the same spacings on 7', 4-5" posts, put 2-3 feet in the ground. The main difference, is that I used good quality hot wire and used offset insulators to keep it at least a foot from the barb fence. Neighbours thought I was crazy. Bulls would be out in no-time as fence wasn't at least 6 feet high. Hasn't happened yet with a small fencer and a good ground rod. Bulls can't judge the height of the main fence because they can't get right beside it with the hot wire in their way.
So in reference, the new perimeter fences are 5 strand barb wire and one offset hot wire. The interior fences are 3 hot wires, although 2 would be sufficient. :cowboy:
If you need pics, I will snap some for you.