I personally wouldn't do more than 6 large butts, 3 on each grate. or 8 smaller butts, 4 on each grate.
And I would definitely do it with my foil wrapped clay saucer diffuser. When all that meat starts rendering, it'll just about put out the fire. The clay saucer will catch most of the drippings, and keep the fire going clean and strong.
Modded Brinkmann Gourmet. Basic UDS. Rescued 22.5" OTS. SJS Mini-WSM. Stubbs. B&B Oak Lump.
Cannondale 2009 F8 (for burning off all the great BBQ)