Five: One to change the bulb and four to pull the ladder out from under him/her.
or
Nearly unanswerable, since the one who tries to change it usually drops it, and the others call for a planning session.
or
Because he wasn't peeling well!
or
Five. Four to decide which way the bulb ought to turn, and . . .
or
"Pint please, and one for the road."
or
Dead.
or