The problem with too many roosters is not just fighting between the roosters (which may or may not happen), but the physical abuse the hens end up taking from the roosters. The recommended ratio of roosters to hens is 1 rooster for every 10 hens. As they mature, too many roosters will become very hard on the hens physically; over-breeding them, biting and plucking the feathers from their necks and backs, battering them, and potentially, seriously injuring. The more roosters per hen a flock has, the more the hens become stressed which affects both their overall health and causes a drop in their egg production. Sometimes with very gentle roosters you can push that ratio a bit, but keeping too many roosters is somewhat like playing Russian roulette. At some point you're likely to pay the price. There are other reasons why chicken owners choose to have roosters, but the only real reason you need a rooster is to fertilize eggs for hatching, and 1 rooster can easily handle 10 hens in this regard.