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
He wanted to win the no-bell prize!
or
Drinking, bathing, washing, swimming, etc.
or
Microwaves.
or
First he lies on one side, then he lies on the other.
or