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.
There were two cows in a field. The first cow said "moo" and the second cow said "baaaa." The first cow asked the second cow, "why did you say baaaa?" The second cow said, "I'm learning a foreign language."