Standard recommendation is for 3 square feet per bird of unused floor space. Much though depends on climate and opportunities to range (as well as the size of the birds).
So 10 x 12 would be 120 square feet, which would house 40 birds, but you'll need to subtract from that space taken up by feeders and waterers, posts that may be holding up your roosts, ditto for nesting boxes, etc.
If you live in a mild climate and the chickens will have access to range everyday, things will go easier.
If they need to be combined to a fenced in run that you create, plan on 10 square feet of space in the run per bird.
If you live someplace where weather will interfere with the chickens being willing to go outside, 3 square feet of space per chicken in the coop may be tight. Depends on the chickens. As an example, we get snow where I live, and my chickens were for the most part uninterested in going outside in the snow. So they remained inside the coop most of the winter; though they certainly had the option of going outside, they weren't interested.
All of these recommendations are generalities. You say "hens" so I suspect that means no roosters, which makes things a bit easier.
Different breeds, or strains within breeds, are more high-strung or more laid back.
Nonetheless, they will need space to be able to get along well. Otherwise there will be issues that will impact how much they lay, and could well lead to other health issues, or injuries if they fight a lot.