None. Agnostics question whether electricity really exists.
or
In the neigh-borhood.
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
Just one, but he'll also want to do something about your nose.
or
A centipede with a wooden leg.
or
Clerk: "Yes, sir. What size does your alligator wear?"
or