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
One. You don't believe me do you? I've got photos! See that big blob? Well, just squint your eyes a bit. . . .
or
Because the teacher said it was a piece of cake!
or
Five. One to screw it in and four to tell him how bitchin' he looked doing it.
or