Keep track as you cook. If you are having a lot of unfertilized eggs - too many hens or more roosters needed.
Sometimes I think that when we think of a coo/run, number of head, count of roosters and hens - What we REALLY think of is our own situations. And really no one keeps chickens just like us. On this forum, there is a huge variance in keeping practices, from weekly vet visits - strict confinement to feral birds, occasionally fed, and everywhere in between. With that wide of variance, a rule cannot be hard and fast.
Keep the least amount of roosters that meets your goals. Know that this number is not a solid number per hens. The age of the rooster, the age of the hens, the weather, the climate, the breeds involved can all shift fertility rates.
I do not think a rooster is stressed by too many hens. They are much more stressed if there are too many roosters. If you are free ranging to the point where the chickens live in trees, well you probably need more roosters. If you have a flock that has a coop and is locked up and protected...you don't need as many roosters.
The more roosters you have- the greater the chance of it going wrong. Personally, I would not want more than one rooster per dozen, and I would not feel the need to add the second rooster if I got up to 20 - but do know that this woman currently has two cockerels in her coop/run.
I like to add chicks each year- some I hatch, and if that doesn't work, I will add chicks. Right now, I have 13 head, and really that is too many for my needs. By next month - I will cut back to 7-8 hens and a rooster. Takes less feed, less water and less bedding, with more than enough eggs. Gives more space in my coop for the long nights of winter. I keep a flock, the number of birds, and the birds themselves changes.
The number is not static, it changes with the needs of the flock.
Mrs K