Five: One to change the bulb and four to pull the ladder out from under him/her.
or
Two. One to screw it in, and one to stab the other in the back and take all the credit.
or
One's a spineless, poisonous blob. The other is a form of sea life.
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
Because he saw the salad dressing!
or
Twenty. One to hold the bulb, two to turn the ladder, and 17 to be on the guest list.
or
T (tea)
or