Some recommend only 2 sq feet per bird instead of 4, however, it also depends on LF vs bantam. But yes if you want 4 sqft per bird then that won't be big enough. 8x12= 96/4= 24 birds
I think it depends on where you live also and the weather. If they only go into it at night just to roost and sleep all yr long then 2 sqft would probably be fine. But if it's super cold outside and they don't want to go out into the run, then they'll get pretty cramped up and start fighting and pecking.
Sometimes the confusing part is that the recommendation is 4 sq ft per bird for standard size chickens and 2 sq ft per bird for bantams. colebarnhart is spot on. If they ever have to be couped up and not able to get out (happens here during really frigid winters) you could have some pecking going on.
I'm a little perplexed too about the square foot thing. But I can tell you that at least in my case it really depends on the individual flock. My gals actually need extra space in order to keep life peaceful, so I'm adding a new little coop to my current little coop. The website said I'd be able to accomodate 5-6 standard hens in the first coop - but it just didn't work. They have a 10 X 10 ft run and an outdoor roost in addition to the coop(s). They just don't like to be crowded.
Are you kidding? I hope so. I do think the Amish know quite a bit about farming. This is probably not really Amish. I'm not Amish either but I can't see saying a whole group of people are stupid...