We would like to have 8 chickens. I want to buy a coop kit but it seems like none of the coops have enough space. Am I missing something?
Here's the thing.
a) there is no hard and fast specific requirement. It is not a matter of simply packing objects into a space. It is a matter of behavior, and what kind of behavior you want the chickens to exhibit, and what chances you are willing to tolerate of various less-good behaviors.
You can most certainly keep chickens with only 1-2 square feet of space per hen, even without a run. That is what commercial battery hens (egg layers) get.
OTOH they are not at all happy, and not especially healthy either, at those space allowances.
Many BYCers feel, for whatever reason (honestly I think it is more 'herd mentality' than anything else, sorry guys) that 3-4 sq ft per chicken in the coop and 8-10 sq ft per chicken in the run is a good amount of space. Certainly it is FAR better than LESS space would be, and at least gives you a pretty reasonable shot at avoiding the worst behavior and health problems.
However I've gotta say, having TRIED having them at 4 sq ft apiece (plus run) and at other amounts up to 15+ sq ft apiece (plus run), giving more space really does make a conspicuous difference in the chickens' behavior. They seem happier and more relaxed and "individual" with the greater space allowance. Personally I will not go back to more-crowded conditions, even if one can often "get away with it" in terms of simply avoiding major cannibalism.
So there is no one magic number. It is a continuum and depends on your personal preferences.
And also,
b) the guys advertising those commercial coops,
they are trying to sell you their coops
Seriously. So obviusly it is in their best interest to quote as high a number as they feel they can get away with for how many chickens their coops will hold. Typically it is based on giving each chicken 8-10" of roost space and packing roosts into the coop. Thus, a 4x8 coop can very plausibly hold 15-22 chickens or so. But how WELL will it hold them? Not too darn well unless you live somewhere that is so year-round every-day mild that the chickens will be spending all of every day of the year outside.
Basically: give them as much space as you can bring yourself to do (= keep as few chickens as you can bear to limit yourself to, for a given size coop) AND do not believe what the coop-selling folks say
Besides, there is really no good reason IMHO to buy a commercial coop, even if you don't want to build something yourself you are still almost always MUCH better off with buying a prefab SHED and converting that. Very easy, just add more ventilation and a roost and popdoor and there ya go
And you'll have a larger (and often less-dysfunctionally-designed) coop for your money.
Good luck, have fun,
Pat