Two. One to hold the bulb and the other to rotate the planet.
or
Lawsuits.
or
Chips AHOY!!
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