@JDann24 So it appears the 18.5" WSM can handle 55# of pork butt. I had 3 on each rack; the 3 on the bottom were laid flat and the 3 larger up top standing on their side kind of in a tee-pee formation. The bottom ones were cooking faster so I rotated 3/4 of the way through.
It was the coldest night in Florida I've ever cooked in, around 25 degrees, which introduced a large factor of uncertainty. I had to reload the charcoal basket twice and the slight wind required more vent adjustment than usual. My temps were all over, after a nap between 12:30-4:30 am, the WSM was at 200. Towards the end of the cook I cranked it up to 300.
With all that meat, my terra cotta heat sink overflowed a little (while removing body to refuel) and caused a small grease fire. Thankfully it was easily extinguished by closing a couple vents.
Overall the cook was successful and a learning experience. Next time I will start 24 hours in advance to provide larger buffer and rotate butts sooner in the cool. Total cook time was approx. 16-18 hours..those bottom butts finished earlier and after rotating the top ones never really caught up.