I haven't loaded the racks yet, but I should think 12 slabs of baby backs would be doable. With the standard configuration on the WSM, I was able to do six slabs, three on each rack, but I had to cut the slabs in half to get them to fit.
So far just from seasoning it, I can tell that it doesn't respond as quickly to damper adjustments as the WSM. I'm probably going to fill the water pan today and do another dry run without meat just to make sure I've got it down. Planning on doing either pork shoulder or some baby backs this weekend; I'm leaving the choice up to the wife.