One makes a lot of assumptions, when they read a post, so I might be wrong. But it sounds like you are first time owners, and may have a disease called chicken math, where as 'this is fun, we should get more...' We have nearly all been there.
My point is, chicks are pretty small, and what seems like a lot of space for chicks and lots of room left over for more birds, can quickly become not enough space for full grown birds.
My advice, is to measure, with 12 full size birds, you should have a coop (building part) that is measuring 6x8, or 48 square feet, the run should be 120 square feet. For 20 birds you should have something around 8 x 10 square feet coop, with a 200 square foot run attached to that.
These are for full size breeds, some breeds tolerate confinement a bit better and you can cheat on the numbers a little bit... but it is good to know just where you are, and a rule of thumb is a good place to start.
If you have taken this into account, I apologize, I mention it, because space looks so deceptive when they are little, and space is the biggest problem in a flock not getting along.
Mrs K