How many chickens that will fit depends on a whole lot of different things. If you follow the link in my signature, you can get my thoughts on some of those things.
I’m not real sure I understand if the 8x12 area is the coop size or the run size. In some ways it doesn’t matter which it is. It’s not the coop size or run size in isolation, but the total amount of space available and when it is available that matters.
If you’ll follow that link and read that, I’ll try to answer any specific questions you have, but there are just too many variables for me to be able to give a realistic answer without knowing a lot more about your unique situation.