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.
Two, one to screw it in, and another to say "Sock it to Me." (Notes: Sock it = Socket. Also, for the infant readers among you, this was a popular catch-phrase from "Laugh In.")