Are they banties or LF? I have an 8 foot long by 4 ft wide and have 5 Buff Orps in it. They fit pretty good, might be able to put one more in there. My other coops a little bigger and I have about 16 banties in it and they don't have tons of room. I'm actually selling 6 of them tomorrow so that will give them more room. I forget the 'ratio' of chicken per sq ft?
I agree with bagendhens for the maximum for standard breeds. Less than 16 would be even better, for more space. A lot depends on your winters, because if your area gets rough weather in the winter season (lots of snow), there's a good chance they will be cooped up for extended days...and possibly weeks. And that is assuming that all of the floor space is useable (elevated nest boxes, hanging feeders, etc.). Some will cram more birds into that space, but over crowding can cause pecking/picking issues. And waste management is easier if there is no crowding.