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
An impasta
or
Twenty. One to hold the bulb, two to turn the ladder, and 17 to be on the guest list.
or
Because he never pays his debts.
or
The letter H.
or
The bartender offers him a warm smile and says, "For you, no charge".
or
Humpty Trumpty wants Mexico to pay for it all.
or