Three - one to cast the bulb into the outer darkness, and two to catch it when it falls.
or
None, because people who glow in the dark don't need light bulbs.
Note: Topical to the Chernobyl Reactor disaster of 1984.
or
Anywhere it wants to
or
Because there's no one else to wag it for him.
or
A porcupine with split ends
or
They cell it.
or