Two, one to call the electrician, and one to mix the drinks.
or
Because an undocumented worker has been living on his head for the past 2 decades!
or
Microwaves.
or
Because it wanted to be a flat screen.
or
None, because people who glow in the dark don't need light bulbs.
Note: Topical to the Chernobyl Reactor disaster of 1984.
or