I always like to think that, if you only have one coop, you can only have one rooster and he is the head of the entire flock. The non-alpha males I prefer to call cockerels. Although the real definition of the word is the same (i.e. a male chicken) it's an important distinction. Important to them too.
Your rooster will tell you when there are too many cockerels, because he'll chase them and beat them up. He doesn't do this until they start showing signs of becoming mature. Our rooster is a relatively amicable boy, he tolerates two younger (but of breeding age) cockerels but has started beating up a large gang of four/five month old cockerels (they are all holders of a one-way ticket to Barbequesville and the bus leaves this weekend).
Assuming that you intend breeding (else you wouldn't need a rooster) you can be sure that at least half your hens will either be sitting on eggs or nursing chicks at any one point in time, the only time the rooster is actively involved is the two weeks of egg laying so factor that into your rooster ratio.
Much as cockerels are lovely to see, too many of them makes life hard for the rooster and worse for the girls - rampaging hormones means that any girls within reach will get chased, jumped, pecked, dragged by the tail or comb and finally forced into submission - for nasty cockerels this includes girls on eggs and with very young chicks - they get a free taxi to Barbequesville before they do any damage.
I'd say one cockerel will do you fine, he'll be a happy boy with no competition and he'll spend more time helping out with the chicks, sounds all very 'new man' but of course he only helps out with the chicks to get the hen back into breeding mode again - not so much 'new man' rather he is a 'smarter old man'.
We have over 40 breeding hens and our rooster services them (for want of a better word) without any problems, he spends most of his day making sure the two young boys don't get a quick fumble with his girls.
ETA - And here he is
