It depends on size or breed, but generally 20-30. Some inside, some outside. provide roost outside. Large breed 20, small bantams 30. Always prepare to make coop area larger, so build with ability to expand. You will always want more chickens.
I am building it so it can expand I knew I would want to get more since I already do want more. I know chickens are little but that 12x8 area seems so teeny! 20 full grown chickens will be happy? It's 6 foot tall so I may build a second "floor" to part of it and maybe plant grass there or something idk.
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.
Ridgerunner is very good about the ins and outs explaining. It has much to do with how you manage your birds.
In a nutshell if you heat in winter your birds will want to stay in the coop. If your coop on stilts then that area is run space but they wont want to hang out in it etc. So many variables that make your unique set up or management style giving you different space requirements you'll need. Trial and error never hurts but a ball park to start with and go from is good too.
The touted number of space per bird in run is what I actually think works very well and that's 10 sqft per bird. Obviously if you had a HUGE run that number would be far to big but in general they like space and prefer to be outside. If one does not heat and keeps food and water outside then large fowl (standard size) only need 2.25-2.5 sqft each in coop. They only roost in there at night with this set up.
So with that said and assuming you have a shed on ground level I'd say you can happily house 17 birds (166 sqft total) and go from there to determine how many more you want to/can fit.
17, 20, 25 in a 8x12 run? Really?
I'm gonna say 10.
I thought most here say 4 sf for coop and 10 sf for run. 8'x 12' = 96 ÷ 10 = 9.6 birds.
I have some 6' x 8' breeder pens that I built for bantams but im using two of them for LF. Mine spend about 95% of daylight hours in the run. One has 4 birds and seems a good fit the other has five and almost seems to small.
Since mine is half the size of yours I couldnt imagine 9, 10, 12 or so birds being happy in there even if a had a huge coop attached.