No, no, no - you're going about it all wrong. You don't have 13 chickens! Here, let me explain it to you...
Here's the math:
13 physical chickens
- 8 because you can't count the babies as they are too young to matter in the coop calculation and some will be cockerels needing to be rehomed.
=5 So that drops your number back to 5 chickens.
-1 And you can subtract that pullet that isn't in the coop because she pecked the babies
=4 that brings you down to 4!
You say you plan to have twelve - well you better get busy because you only have FOUR! hahahaha