A good ratio of hen to rooster is 8 to 10 hens per 1 rooster.
Introduce new birds slowly to a flock. And never mix in chicks younger than 3 months of age. If you are mixing in a rooster, if he is younger than 6 months of age, hens can be aggressive toward him. And try to always integrate more than one bird into an existing flock. One bird becomes a huge target. Two or more dissipates the aggression.
Keep the new birds in a cage or enclosure within the run and coop for about 3 to 4 weeks. Everybody sees, nobody touches. This gives everybody a chance to get to know each other. After about 3 or 4 weeks, you can mix them all together. Watch for too much aggression the first week and always intervene if it turns bloody. Put out more feeding and watering stations too as the original flock can starve them out.
Keep your space requirements of 5 square feet per bird in the coop at all times and 10 square feet per bird in the run as well.