If smoking nine St. Louis slabs, you can cook 4.5 in the bottom, and 4.5 in the top, using racks. I've only done this once, and actually only four were trimmed, but I want all my ribs exposed to smoke and heat the whole time. I use the flimsy Charbroil rib/potato racks.
I don't know if you can, but if they weren't too big, maybe you could roll four on the bottom and put five on the top with the rack. It only has three slots, but you can lean the other two up against the outsides. I just like to keep the ends away from the outside of the bottom grate if I can.