Two. One to change the bulb, and one to write a song about how good the old light bulb was.
or
None, because people who glow in the dark don't need light bulbs.
Note: Topical to the Chernobyl Reactor disaster of 1984.
or
The police had to comb the area.
or
Tell it a yolk.
or
Because there's no one else to wag it for him.
or