Fifty — one to screw in the bulb and 49 to guard him/her.
or
None, because they were copycats
or
Two. One to take notes while the other screws it into the faucet.
or
None, because people who glow in the dark don't need light bulbs.
Note: Topical to the Chernobyl Reactor disaster of 1984.
or
"Let me see your birth certificate".
or