Only one, but it may take upwards of five years for him to get it done.
or
Three. One to climb the ladder while the second kicks the ladder out from under her. And the third to say, "I knew that was too high for you dear."
or
......... one jar.
or
Time is fun when you're having flies
or
Two. One to take out the bulb and drop it, and the other to try and sell it before it crashes.
or
Fifty — one to screw in the bulb and 49 to guard him/her.
or