100: One to do it and ninety-nine to say "I could've done that."
or
The barman says "I'll serve you, but don't start anything"
or
If Trump gets Alzheimers his IQ will go up.
or
A screwdriver.
or
Two. One to change the bulb, and one to write a song about how good the old light bulb was.
or
1,000,001: One to change the bulb and 1,000,000 to rebuild civilization to the point where they need light bulbs again.
or