If he is used to service many mares, the breeder can run the risk of having a healthy percentage of the mares come up open. Of course the stallion will, more than likely, get the bad rap of being a infertile. Obviously that is the case. It is probably due to over use rather than some organic problem with the horse, however. It certainly is a lot better to produce a few foals the first year or two than to run the risk of setting the rumor mill running about the incompetence of your prized stallion.
Veterinarians can run a fertility check on stallions by collecting a sample of sperm.
Those of you who are considering standing a young stallion, may consider having a fertility evaluation performed. At least, it would be useful to obtain guidance from a qualified veterinarian as to their recommendations for breeding the young stallion.