Only one, but the bulb has got to really WANT to change.
or
Twenty. One to hold the bulb, two to turn the ladder, and seventeen on the guest list.
or
It ran out of juice!
or
100: One to do it and ninety-nine to say "I could've done that."
or
Your wife comes back to you, your dog returns to life and you get out of prison.
or
In the dictionary.
or
Bozo The Clown has real hair on his head.
or