Just one. To threaten suicide if you don't change it for him/her.
One....Two...One, Two...
It doesn't matter, mine itches so bad it's on fire, providing plenty of light for everyone in the room!
Three. One to change it while the other two argue about how old the old one is.
None. They live in eternal darkness.
A: Only one but it may take him/her more than five years to do it.
One to change the lightbulb and one to drive down to Kent to pick him/her up.