Here goes. It depends.
Area=Pi x Rsquared. So if you have the radius you can figure the cooking area. Now not all chickens have the same size thighs. Not all cooks have their birds touching or equal distance apart on the grill.
Now I've gone and done it. My brain hurts. Should have just stuck with It depends. Its a sorry excuse of a reply, but truthful. And I would guess 8-10
Primo Oval XL, Tiernan SOB, The Pit Barrel Cooker, BGE-Mini, Blackstone Griddle in current rotation