The previous poster explained the space requirements quite well. So I will address the chicken math issue.
You start with 4 chicks, and one "fails to thrive" so you only have 3 chicks. Well, that's sad, must go replace it. At the feed store, you realize you cannot buy just ONE chick - that would be so awful for that chick on the way home. So you have to get two. And if you're getting two, and you had four in the beginning but now are down to three, you KNOW you can get four because you already did that the first time. So you come home with four.
Now you have seven chicks. But really, one doesn't count because it's just replacing the one you lost. So you REALLY only have six chicks.
That's how chicken math STARTS. Like all mathematics, there is more than just simple addition. There is subtraction, multiplication, and algebraic calculations. Bantam breeds only count for half a chicken because you can have two bantams for every large fowl chicken. Chickens somebody gives you don't count - you didn't buy them, right?
And so on, and so forth.
It's how I started with the intention of having 4, maybe 6, no more than 8 chickens...... and you can read the result in my signature paragraph.
....plus I bought an incubator and just yesterday hatched six little chicks from 7 olive egger eggs I won in a BYC auction.... one drowned in the incubator water reservoir, so of course I'm gonna have to get more eggs to hatch.... can't put just ONE egg in an incubator and expect it to hatch, right? Need more, to ensure enough hatch....