Seven. One to change it and the other six to sing about how good the old one was.
or
As many as possible, and don't *ask* what they do with the old bulb.
or
None. They just let Marketing explain that "Dead Bulb" is a feature.
or
Barium
or
The scales.
or
They can't find the key, and they never know when to come in.
or
By carving a Trumpkin.
or
Because they had so many knights.
or