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.
Rescued 22.5" OTS. SJS Mini-WSM. HEB briqs (rebranded Royal Oak). Pecan/Hickory/Cherry/Apple/Peach woods.
2016 Fuji Absolute 2.0 LE (for burning off all the great BBQ)