What size of chicken? For full sized chickens, the normal ratio is around 10 to 12 hens for each rooster, but for bantams the ratio is more like 12 to 15 per rooster. Age and vitality enters into it too. A younger rooster will do a better job of mating with the hens. Health an nutrition also play a part. At commercial hatcheries, they use a ratio of around 10 to 1 and replace older roosters when fertility drops.
It is not that a rooster mates with each and every one of his hens every day. After a mating, hens normally stay fertile for about 2 weeks. Some less and some more.
Often one rooster can keep 20 hens fertile, especially a young, healthy, energetic rooster. But sometimes one rooster will not keep 5 hens fertile. That's where the average ratios come in. The good performers compensate for the poor performers.