You need to know how to do the math so you can figure it out on your own for any size coop that you might think about building.
Adult, standard size, (not bantam) chickens need four square feet of floor space for each bird.
If the floor is 4 feet long by 4 feet wide, just multiply those two numbers to get the square footage. 4 X 4 = 16 square feet.
Your coop's floor is 8 feet by 8 feet.
If the coop floor is 8 feet long by 8 feet wide, that's 64 square feet. 8 X 8 = 64
If the coop floor is 4 feet wide by 16 feet wide, that's the same floor space as 8 by 8. 4 X 16 = 64.
If the coop floor is 3 feet wide by 21.33 feet wide, that's also the same floor space as above. 3 X 21.33 = 63.99 square feet. (Less than an inch smaller than your coop.)
Once you know the coop's floor's square footage, just divide that number by 4, to get the number of birds that can fit in that coop. (Because each standard size breed adult bird needs four square feet of space.)
So a 64 square foot floor can hold, at most, 16 birds. Those are basic minimum space requirements. (More space means less squabbling. )
Adult bantams are smaller birds and don't need quite that much space.