Three - one to cast the bulb into the outer darkness, and two to catch it when it falls.
or
A whole synod. One to move that the bulb be changed while the others debate until the room spins.
or
Two. One to exploit the proletariat, and one to control the means of production!
or
None, because they were copycats
or
Three. One to climb the ladder while the second kicks the ladder out from under her. And the third to say, "I knew that was too high for you dear."
or