I started off being immune to chicken math. I'm such an amateur.   I'll need help.
I ordered 15 chicks last spring to arrive april 18th.  I resisted the urge to buy "just a few" chicks from 
tractor supply during chick days to hold me over til the other chicks arrived. (That was REALLY hard.)   Tornados in the midwest caused a shipping delay, and I only got 3 live chicks (so sad).  The hatchery replaced the entire order but one didn't make it, so I ended up with 17.   
I figure I have white eggs, and light brown eggs, so I need blue eggs and some marans eggs.  Maybe some Welsumers.  For variety, you know.  
  One dominique was lost to a predator, and the rooster got too mean (He was a hatchery sexing mistake).  Since chicken math is exponentially driven, and I need to replace 2 chickens, I will get 2 black copper marans, and 2 welsumers.  And since you can never tell what color eggs you'll get from an easter egger, I will have to get at least 5 of those to make sure I get at least SOME blue eggs, right?
I think I've shown amazing self-restraint.  
On a more serious note,  I have an 8 x 16 coop, and a 10 x 30 run.  The girls get out every day for at least an hour to forage.   The forage is GREAT (we're in the middle of 300 acres of mixed woods with an old cow pasture with clover, dandelions, and other wildflowers with LOTS of bug variety).   I think we'll expand the run at some point, but the coop can't be expanded.   I don't truly know how many I will be adding.   I will see how they do this winter with the amount of floor and run space they currently have.  I don't want to add too many birds and make them miserable.   I know people have more birds in a smaller space, but we have a LOT of property, and the girls have a good life here.  I don't want to cramp them because of my own need for more chickens.