Two. One to take out the bulb and drop it, and the other to try and sell it before it crashes (knowing that it's already burned out).
or
It's what the queen says after she burps.
or
Three:
One to write the light bulb removal program,
one to write the light bulb insertion program, and
one to act as a light bulb administrator to make sure nobody else tries to change the light bulb at the same time.
or
To give us some sort of appreciation for flutes.
Did you hear about that music composer who committed suicide? He didn't even leave a note.
or