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
The sound of Mew-sic
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 sometimes you can get rid of cancer.
or