Seven. One to change it and the other six to sing about how good the old one was.
or
Two, one to call the electrician, and one to mix the drinks.
or
1,000,001: One to change the bulb and 1,000,000 to rebuild civilization to the point where they need light bulbs again.
or
Because he wasn't peeling well!
or
He was in shock for a week.
or