You can put a bunch of butts on the WSM. It just depends on what steps you are willing to take. If I split them and use an extra grate, you can fit 8 butts into the cooker with ease.
Anyway back to 4 butts. This is the number that I most regularly cook in my WSM and I do them whole. I actually just did 4 butts on Saturaday night. (1 with Head Country Rub, 1 with Byron's Butt Rub, 1 with Salt Lick Rub & 1 with Blues Hog Rub.)
I started a fire using the minion method and tossed some apple wood chucks. I used an EMPTY water pan. I set the BBQ GURU at 250 degrees, at the top grate, w/ a meat temp of 200 degrees and used the ramp mode.
BTW, the gentleman above is correct when not using water, the butts on the bottom grate will always finish first. I put my larger butts on the bottom grate.
The meat went on at 11pm... At 1pm Sunday afternoon, I took the butts off the cooker. They were all between 195 & 205 degrees. BTW, they were the best butts that I have ever cooked.