Three, One to climb the ladder. One to shake it. And one to sue the ladder company.
or
None, because people who glow in the dark don't need light bulbs.
Note: Topical to the Chernobyl Reactor disaster of 1984.
or
By paying them to cheer for him during campaign events.
or
10. One to hold the bulb and nine to rotate the ladder.
or
Its days were numbered.
or