you need atleast 3 sqft of space per bird..... so 21 but more comfortably 4 sqft per bird is 16 birds if you have a small run i would go with the later number.
i have 30 but its cramped although i have a 400 sqft run for them as well.
The "standard" thought is that you should have 4 square feet per chicken. Some decrease this to 2 square feet per chicken if they're bantams. What type of chickens are you hoping to get? Including a roo? Will they have a big run or free range? The usual recommendation for the run is 10 square feet per bird.