Prefab manufacturers lie like rugs, take any number they say their coop can hold and a third of it is the real number.
Nothing sours a new chicken keepers experience quicker than a prefab, always go bigger. A good hand-built tractor will always have a buyer.
I have an 8x8 for 13 birds, and I...
Depending on the model, I've never seen one that could hold more than 4 birds, maximum.
They cost $1000, you can most certainly build, or find someone to build you a decent sized tractor, If you're unable yourself.
Here are some plans...
Unless you're absolutely set on an omelet, I would recommend you cancel your order and spend that $$$ on having someone build you a coop.
Omelets are lacking in space, ventilation and aren't predator proof like almost all prefabs.
They aren't all they're cracked up to be.