Fifty — one to screw in the bulb and 49 to guard him/her.
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
Weeeeeeeeeeeeeeeeeeeeeeeeeee
or
A bed.
or
Flood lights!
or