I have 15 together with no problems, but if you add a new roo be ready for the poor thing to get hit on for a while till they get use to him, also in the late evening the roos somtimes try to breed each other, but mine fair out ok.
i keep oeg's for show birds so i always have lots of roosters, i keep them together all the time with no problem, quite often i'll have hen's with them too and still no fighting.
I figured if no hens no issues. I have three in the big pen now and they seem to get along pretty well until one of the younger ones tries to mount one of the hens.