Lots of good reading on BYC. Good idea to have the coop ready before the chicks arrive.
How many eggs do you want a day? Since chickens don’t lay an egg everyday (with the exception of production birds), you may find you need 10 birds to always get 6 eggs, for example. We have 10 layers, we get 7/eggs per day usually (range 4-8/day). One of our hens is broody again, so she’s not laying.
So, when you determine you number of birds expected in your flock, then make sure you build enough space inside the coop and in a protected run. Even if you free range outside the run, you’ll find it helpful to have a secure run when predators are around.
Where are you located? Hot, cold, dry, wet? All these factor into coop design and choice of chicken breed. Choose less heavily feathered birds if you live in a very hot place, likewise choose breeds known to tolerate cold well if you are somewhere that can have long or intense winter weather.
Check out the article section on BYC too, in addition to the forums.
Good luck!