You are getting some good advice here. There are so many differences in our management techniques, climate, flock make-up, and goals that there are no magic numbers of square feet per chicken that fits us all. We are all unique and have unique requirements.
Don’t buy any pre-built coop or use a coo kit until you post a link to it on here and get comments. Many of them are poorly made, use cheap materials – especially hardware, are not really designed for chickens – they take something for pets add a roost and nest and say it is for chickens, and they practically never house anywhere close to the number of chickens they say they can. There are a few decent ones out there but very few.
You are practically always better off building your own if you are looking for more than about four hens. Some, not all but a few, of those could work for four hens but not eight. Even then you normally have to increase the ventilation. I think AkChris gave excellent advice.
You can follow the link in my signature to get some of my ideas about room, and I suggest you read these articles before you select a coop to build and where to build it.
Pat’s Big Ol' Ventilation Page
https://www.backyardchickens.com/web/viewblog.php?id=1642-VENTILATION
Pat’s Big Ol' Mud Page (fixing muddy runs):
https://www.backyardchickens.com/web/viewblog.php?id=1642-fix-a-muddy-run
For twelve chickens my minimum would be a coop that I can walk into and stand up comfortably. If you build it so that you have enough room to comfortably work in there with nests and roosts it will be big enough for twelve. And since most new building materials come in standard 4’ and 8’ dimensions you can usually save yourself a lot of cutting and waste if you take that into account in your design. It gets more expensive to provide enough room but no one ever complains about having too much room. And as Bee said, too little room can lead to bullying, picking feathers, cannibalism. Can. It’s not guaranteed but I find the tighter I pack them the more behavioral problems like this I have to deal with, the harder I have to work, and the less flexibility I have to deal with issues. Go with something big enough to make your life easier.