Since we love left-overs, I put 8 butts on the smoke Friday night. I used a simple, but proven rub, but skipped the brown sugar since my wife can't eat it. Instead, I basted the butts heavily with melted raw honey. A huge mess, but the end result was great.
Only issue is that with 8 butts on the XL, using the Ceramic Grill works 2-level grid, they have to touch. I had 5 on the bottom and 3 on top. I found that the bark was not as good due to restricted circulation. I mixed all the meat together and everyone loved it, but just not the exact same.
I think the magic number is no more than 6 - 4 on the bottom and 2 on the top.