This mostly depends on the amount of hens, and space you have in your run. Often two roos will share rooster duty once they have established a dominate rooster. Your flock will have a few squabbles with any new introduction, especially a new roo. As long as fights don't draw blood it is best to let them adjust on their own. It can look quite violent even without bloodshed, this is how they establish who is in charge. If the rooster being introduced is younger he probably won't even try to challenge the head ro until he is full grown.
A good rule of thumb is to wait to introduce new birds until they are about the same size as your existing flock. If you can't wait, just make sure the smaller birds can hide if things get too aggressive.
