Because an undocumented worker has been living on his head for the past 2 decades!
or
Two, one to call the electrician, and one to mix the drinks.
or
On the latest episode of "Celebrity Apprentice", Donald Trump fired himself!
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