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
No one knows. Republicans automatically disbelieve him, and no one can ever trust a stinking liberal anyway.
or
One; she designs the bulb to crawl up the wall, unscrew the old one, and screw itself in.
or