To calculate your cooking time, I use 1.5 to 2 hours per lb of the typical size of one of the butts, if cooking in the 225 - 250 range. For instance, last year I did a cook of 6 butts that were about 7 - 8 lbs each, so cooking time was estimated at 14 hours. They were actually finished in less than 12 hours.
My tips:
1. Allow enough time to pull and shred the pork as it takes longer than you think. I think it took a 1 - 2 hours to pull 6 butts and chop it.
2. Paul was correct in that it will take more fuel to cook the 6 butts. So have a 20 lbs. bag ready.
3. Get creative with arranging the butts in the 18.5". You may need to stand them on end to get all 6 of them to fit.
4. Get 2 large disposable pans for the meat. I think I ended up with a little over 24 lbs of meat, which completely filled two large pans.
5. Let the WSM do the work. It was no more problem controlling heat with 6 butts as it is 1 butt. Typically when cooking 1 butt I rarely open the vents more than a 1/4 open. For 6 butts, vents were left 1/2 open or more for the entire session but temp was steady for hours on end.