I've always heard 1 roo to 10 hens but as mentioned I'm sure that different breeds will make that number vary. I had 1 roo to 10 hens (before I lost all my girls and their 10 babies, a couple of weeks ago). My chickens free ranged during most days and locked in at night. I never had a problem with him picking on any, he just seemed to make sure he did his duty on each one, everyday. There were a couple who didn't like him so they always went in the coop last to avoid him, funny thing was, those were the two who went broody. Out of the 10 hatched I had 3 cockerels that I was going to find a home for.
I think that if you never let your chickens out of their coop/run then you may need to manage the roo more but I never had problems...... but my roo is a sweetheart.