Twenty. One to hold the bulb, two to turn the ladder, and 17 to be on the guest list.
or
Five. One to change the bulb and eleven to say they could do it faster.
or
Seven. One to change it and the other six to sing about how good the old one was.
or
It held up a pair of pants.
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Only one. But he'll leave a big puddle of spit on the floor underneath him.
or