Are you starting a breeding program? If so, you really outta have his fertility tested, because it's not uncommon to find abnormalities that affect it. Set up a breeding soundness examination first or you could be wasting time and money. I personally think 25 cows would be the uppermost limit for a yearling bull. I like to start at 15-20 cows for the first breeding season. I up that number after the first season and the amount really depends on the bull. That's just a personal preference though.