He wanted to make a clean getaway!
or
None, because people who glow in the dark don't need light bulbs.
Note: Topical to the Chernobyl Reactor disaster of 1984.
or
Two, one to call the electrician, and one to mix the drinks.
or
Two. One to screw it in, and one to stab the other in the back and take all the credit.
or
Because 7 ate 9.
or