Two. One to assure the everything possible is being done while the other screws the bulb into the water faucet.
or
Weeeeeeeeeeeeeeeeeeeeeeeeeee
or
The owner got tired of the (w)hole business!
or
He presses the paws button.
or
He was a baaaaaaaaad driver.
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