Oh, enough with the "accepted rule of thumb" stuff, everybody! Aaargh! <tears hair>
Sorry, I am calmer now
Look, how many chickens could (or more appropriately to ask, "should") be in a given space DEPENDS GREATLY ON THE SITUATION. It makes me nuts to see this 4 sq ft number quoted over and over and over like religious dogma.
You gotta realize, it is just a BYC convention, something that has got popular on this site. As other longstanding groups of people, get different numbers.
At best it is a VERY GENERAL BALLPARK for what may help you avoid the worst risk of cannibalism, in a moderate climate.
It really is not a good guide for many INDIVIDUAL people though, as their climates and personal goals may easily be quite different.
If you live somewhere always-beautiful where hcickens never want to stay indoors for the day ever, 4 sq ft per chicken is considerably more than needed, all they need is enough length of protected roost.
If you live somewhere with many days when your chickens will want to stay inside, and/or have genetically-grumpy chickens, and/or are aiming at something a little more than "don't attack each other", then 4 sq ft is considerably insufficient.
Personally in northern Wisconsin, which is a more severe winter climate than we have here an hour north of Toronto, I would not put more than about 8 chickens in there. If the o.p. is doing well with her existing 12 chickens then that is great, no problem there, but i would REALLY not advocate getting more. Yeah, you might get away with it. But you also might well NOT, and you are quite likely to see much more stressed chickens even if they do not *actually* get to the point of cannibalism.
JMHO,
Pat