You don't get guarantees with living animals and their behaviors. It just doesn't work that way. Each chicken is an individual with its own personality. Each flock has its own dynamics. Even in the same circumstances different birds will react different ways. The different personalities of the birds it is housed with will affect how it reacts. We don't house them all the same. How much room they have, how that room is laid out, and how they are managed can have a big effect. There can be a big difference due to maturity/age differences. For all these reasons there is not magic number for hen to rooster ratio.
That often quoted 10 hens per rooster comes from hatcheries that use the pen breeding method where they may have about 20 roosters in a pen with 200 hens. They have found that with full-sized fowl a ratio of approximately 10 to 1 pretty much assures fertility of all eggs. They don't blindly use that ratio, they monitor fertility and add another, usually younger, rooster if fertility drops. At some point they remove older roosters so they aren't paying to feed more roosters than they need to. With bantam flocks the right ratio for fertility may be between 12 to 15 hens per rooster, again they check fertility. Most of us don't use the pen breeding method therefore those ratios don't have much meaning for us. We manage them differently.
When I was growing up Dad had a free ranging flock of one rooster and around 25 to 30 hens. Practically every egg we put under a broody hen developed and hatched. The fertility rate was very good. But Dad replaced that rooster before he got too old, maybe 4 to 5 years of age. A younger rooster is usually more active than an old one. Some roosters are more active than others, some may only keep a half dozen hens fertile. Again, a lot of variables. You can check for fertility by cracking an egg and looking for the bull's eye. This thread shows you how. You may need to gently turn the egg over if the blastoderm is underneath.
Fertile Egg Photos
http://www.backyardchickens.com/t/16008/how-to-tell-a-fertile-vs-infertile-egg-pictures
Sometimes you can keep a flock with multiple roosters and things work out fine. Sometimes the roosters will fight to the death over flock dominance. The number of hens doesn't matter. If they are going to fight over the girls they will fight whether there is one or twenty. Adding more girls won't change that, you just have the added problem of integrating new chickens. How much room they have can play a big part in how they interact, the more room the better. Immature pullets and cockerels often behave quite differently than mature hens and roosters. If you can get them through the juvenile phase and provide a lot of room your odds of it working increase. But even if they totally free range with no fences and separate coops you might still have problems. Probably not but could.
We want to make sure that there is no unneeded stress or heath issues with this ratio. Can any of you suggest anything to make this better? We are planning to buy more hens. How many do you think we need to make sure they are all happy. Do we need any more hens?
This is your question. The reason you may need more hens has nothing to do with making them happy, it depends on your goals. If you need more or certain breeds to meet your goals that would be a legitimate reason to add more hens.
I don't know how old yours are right now, how you have them housed, or how they are behaving. The best thing you can do for behavioral issues and health reasons is to give them as much room as you can. The smaller the space the heavier the poop load which can cause health issues. Poop management is important. The more room you can give them the less likely you will have to deal with behavioral issues. Packing them tightly makes any behavioral issues worse.
Again, not sure how old they are or how they are behaving. Especially if they are still fairly young I'd strongly suggest you prepare an area where you can isolate one or more of those boys. If they haven't made it through puberty things can go pretty wild pretty fast. If you plan on breeding them you will need breeding pens anyway as Aart mentioned.
