The ratio depends on whether or not you're free ranging and the breed. High-strung, flighty breeds that are kept in a coop and run all the time need a higher ratio (10 or 15 to 1), if you have more even-tempered birds and you're free-ranging constantly you can afford to have a lower ratio.
I always like to have redundancy, especially if you're free-ranging at all. Even if you don't end up with an aggressive roo there's always the chance of a predator attack or random injuries or diseases.
When I decided to get roosters, my original plan was to get six of three different breeds and keep the best three.
Last April I got six roosters to add to my free-range flock, with the intent of only keeping 3 or maybe 4 depending on how flock dynamics worked out.
I ordered 3 different breeds: 2 cream legbar (5 of my hens were legbars, and 2 are olive eggers with legbar genes), 2 blue cuckoo marans (a couple of my olive eggers are part cuckoo maran), and 2 blue copper marans (my favorite olive egger hen looks to be part blue copper maran).
I was looking to breed for colored egg-laying and more active birds suitable for free-ranging.
Things kind of went off the rails from the get-go: One of the legbars was substituted with what I thought was another blue cuckoo maran. Later on it turned out to he was an olive egger they put in by mistake (Meyer did refund me on that bird). My cream legbar developed curled toes on his right foot, earning him the name "Vulcan."
He's a sweetheart and gets around quite well in spite of his handicap, but he's at the very bottom of the rooster pecking order because he can't really run or fight, and I think he's either sterile or he his bad foot prevents him from "hooking up" properly. To date he's fathered no chicks but he's a good watchdog of the coop and run.
The two cuckoo marans gave me the most problems. The bigger one developed cardiovascular issues, and I had to euthanize him at three months old. The other one got borderline aggressive at six months and I thought about culling him but I was able to get him calmed down. Getting knocked down the pecking order a month ago did wonders for his attitude. He's practically a pet now.
The two copper marans run the show now, and are pretty even-tempered. The only thing negative I'd say about them is they're loud. The like to crow and the they're both Chatty Cathys: they constantly cluck and burble while they're going about their daily business. They remind me of those old electric coffee percolators.
The big surprise was the olive egger, who turned out to be the best protector in the flock: I think he covers more ground than all of the marans combined in a day. He's also very smart about sounding off warnings: yesterday we had a daylight raid attempt by an opussom. I spotted it from my window jogging out of the woods towards the chicken coops and just as I got up to go outside the OE hopped on top of the outdoor brooder and started flapping his wings and raising the alarm. At which point the opussum turned tail and ran back into the woods.
The olive egger and the legbar also acted as "foster dads" when I introduced new chicks to the flock this past fall: keeping the hens from bullying them too much and showing them where to hide from danger and the best foraging spots. I don't know if it's breeding or just those two.
Interestingly, the olive egger has managed to fertilize about half of the eggs I've hatched from my hens so far even though he's maybe #3 in the rooster pecking order.
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I think he's just ready for us to get them so I'll shut up about them 
