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
It's not a shrub, it's a lemon tree my dear Watson.
or
Because he never pays his debts.
or
I rang her up, I said "Did you get my drift?".
or
None. The darkness will cause the bulb to change by itself.
or