Four. One to change the bulb and three to whine "It's too high"
or
Because the chicken was on vacation.
or
They are triplets.
or
None, because they were copycats
or
A blackboard.
or
Two — one to change the bulb, and one to write a song about how good the old light bulb was.
or
A taxi driver.
or
None. The light bulb contains the seeds of its own revolution.
or