I don't have much experience when it comes to this type of stuff, but here I go.
I wouldn't add any more roosters then you need to. All they'll do is fight. If you want to pull some out and replace them. I'm not sure how that would work out. Just make sure whatever you're putting in is around the same age, if not, a little younger then the rest of the females.
I think a great deal depends on your bachelors. I had 20 cockerels in my "butcher pen" I had a few trouble makers and once I got rid of them it was basically peaceful, didn't matter who came and went. That being said, I never added completely new birds; just rotated the same ones. I'm interested to see what others think!
Had me confused
When adding new roosters to a bachelor group, try to rearrange thinks like feeder and waterer position and whatever structures you have in the pen. Objective is to get so established roosters think they are in a new situation. This should increase fighting among the established birds. While this fighting is going on add the new roosters. They will get into the fighting as well but they will not be targeted by an established group that will attack them everywhere they go.
Adult roosters are more problematic. Once you start talking bachelor pens, more birds are better assuming they are not overly confined (area per bird too small). Also introducing multiple new birds better than doing one. Adding a fulling mature rooster in with younger males can have a stabilizing effect. Do not do with with game chickens.
Free-range setting will allow losers to get away although you will have trouble getting them to return to provided roost the following evening. I have played around with confined and free-range introductions and found on the whole the confined approach is better. It does require some supervision. I would also do it later in the day and rearrange roost sites as well otherwise low ranking birds will be sleeping on the ground.
Does size and age make a difference? Will it be easier to add the younger ones?