A general rule of thumb is 10 hens to 1 rooster, the reason is you want to make sure you have enough hens to satisfy your roosters manly...desires.
Lets say you have a very active rooster with five hens (like I did at one time), it's pretty likely you'll have a few with bare backs and all hens in bad condition. For breeding purposes, I know people have "trios" (1 roo, 2 hens) and "pairs" (1 roo, 1 hen) which is fine, you just want to make sure the rooster is not too hard on the hen(s). If he is hard on the hens, just leaving him with his girls for a week or two to ensure fertilized eggs might be an option for you.
However as with most things, there are exceptions. Some roosters (few and far in between, in my experience), are very gentle on the hens and keep mounting to a minimum. For two of these "gentle" roosters I would suggest anywhere between 12-15 hens to keep them both happy.
For two "average" roosters I consider the 10 hens to 1 cock rule ideal. So 20 hens to 2 cocks. You might be able to get away with 12-16 hens to 2 cocks. Just a preference. They are your chickens and you have a right to do whatever you want with them.
And yes, if you get a rooster then later add another rooster, there will be a cock fight. I once had an EE roo and a bantam Blue cochin roo (very sweet) that got along together famously b/c they were raised together. One would take 1/2 of the hens and watch while the other took the other 1/2 and watched them - super adorable!
So raising two cocks together would be a great option and result in a happier flock IMO, then adding another roo would when he's in full bloom.
Good luck!
