It does sound sadistic. It probably made them panic, "OMG! I only rehearsed 6! Have I forgotten to learn and rehearse one?" It is a good joke, if you are not on the receiving end of the joke! LOL
I'm sure that he was waiting in the wings, telling them "Three more to go" then "two more to go" and so on. Like boot camp in the Marines, when soldiers' morale is tested, i.e., after completing 100 push-ups, they are commanded to do 100 more.