LoL the part they sell is a pack of 4; if you input qty 6 into the little box on the webpage i'll bet you'll get 24 spark plugs in the mail
Again if these dudes are selling 4 plugs for a bike that needs 6, they're either not tuned in to what the Rocket needs on a fundamental level (which means they can't be trusted in their indication of these plugs being good for Rockets), or they want you to buy 2 sets (8 plugs) in order for you to get your requisite 6 hence more $$ for them.
Either way i wouldn't be buying off them