A Chair!
or
Two… one to tell the engineer to do it, the other to say "I don't know, what do you think?"
or
One has claws at the end of its paws and one has a pause at the end of its clause
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