Last summer I did 6 8-10 pounders for a dual graduation party I co-hosted. It was a little tight, but still had room to close the lid. I smoked a little lower temp wise(225-238) and with a full water pan. Looking back at my post from last year, it appears the session went almost 22 hours, and I had to do a full chimney fuel refill 3/4 of the way through.
I JUST did 4 butts last weekend (two for a beer tasting my brother was hosting, two for my family) and still had plenty of room for my Maverick temp guage that I usually have on the second shelf.
Also have done 4 full size racks of spares, and two chickens on a 18.5 as well.
You should have no issue with 4 butts on a 18.5.