If you are cooking for any crowd, especially a big crowd, start early. Build in a few extra hours in case you have any hiccups along the way. Butts can be wrapped in foil and a clean dry towel and held for hours in a dry cooler and will still be hot when pulled. I've held for 3 hours doing this and others have done 5+ hours.
I have not done multiple butts on my 18 yet but i know people do 4 butts on them routinely, although i'm not sure of the size butts they're cooking.
If you are cooking a large quantity, keep the following in mind: 4 butts will likely be 30-40 lbs of cold meat. It might take a cooker a bit to come up to temperature with so much cold mass so schedule accordingly. Taking the butts out of the fridge early and letting them come to room temperature will help with this.
Cooking temperature is a personal preference but i and many others do butts at 275 which will speed up your cook time a bit. again this is all personal preference.
I have a buddy that does lots of bigger cooks on a UDS and always cuts the butts up into pieces to get more surface area and thus faster cooking and more bark. I did this once and got very positive feedback. I don't usually do it but its an idea.
Keep an eye on the temperature differences between your grates, you may opt to rotate the butts top to bottom if the top is hotter to even out the temps. as it is, every butt will cook at its own speed.
Good Luck with the cook, keep a log and take photos so we can hear about the cook.