A cow walking backwards
or
Three - one to cast the bulb into the outer darkness, and two to catch it when it falls.
or
Three:
One to write the light bulb removal program,
one to write the light bulb insertion program, and
one to act as a light bulb administrator to make sure nobody else tries to change the light bulb at the same time.
or
A multiplication table.
or
he got a little behind in his work.
or
Twenty. One to hold the bulb, two to turn the ladder, and 17 to be on the guest list.
or
Only one. But he'll leave a big puddle of spit on the floor underneath him.
or