Fifty — one to screw in the bulb and 49 to guard him/her.
or
The leaning tower of Pizza.
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
A ruler.
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A blackboard.
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One's a spineless, poisonous blob. The other is a form of sea life.
or