Two. One to take out the bulb and drop it, and the other to try and sell it before it crashes.
or
"Pint please, and one for the road."
or
Seven. One to install the new bulb and six to figure out what to do with the old one for the next 10,000 years.
or
One minds the train, one trains the mind.
or
Only one, but the bulb has got to really WANT to change.
or