One; after reflecting in the twilight on the merit of the previous bulb.
or
Two. One to assure the everything possible is being done while the other screws the bulb into the water faucet.
or
100: One to do it and ninety-nine to say "I could've done that."
or
None, because people who glow in the dark don't need light bulbs.
Note: Topical to the Chernobyl Reactor disaster of 1984.
or