Well, depends if you go by commercial standards which allot about one sheet of paper's space per bird, prefab coop standards that allot anywhere from 2-4 square feet TOTAL between coop and run per bird they *say* you should house in them, or the more commonly accepted general rule of thumb that is to allow 4 sq ft inside and 10 sq ft outside per large fowl bird (it can be adjusted down for bantams if you really feel the need) -- but, ultimately, rather than looking at any space and asking yourself how many birds you can shoe-horn into it, it's best to consider "how much space can I give my birds" because there really is no such thing as giving them too much space, and many negative outcomes that can come from giving them too little.
Climate plays a big part in the needs of your flock - a climate that is frequently inclement and would encourage birds to stay indoors require more space inside - climates that are more likely to see the flock outside for the majority of the year, using the coop for roosting and egg-laying only makes the amount of space inside less important than the space outside.
I provide one nest box per every 4 hens - with a minimum of 2 boxes for any flock over two birds.
