Like our brethren said.... cooking one or cooking 6 it's all the same. Just plan a bit more time as that much "cold meat" will take longer to come to temp. BUT... don't rush it. Let that cooker rise in temp slow to allow the butts to bath in tasty smoke flavors.
Cooking on a Kamado, Weber Performer, Hibachi & MSR WhisperLite (my only gasser).