Only one. But he'll leave a big puddle of spit on the floor underneath him.
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None. The light bulb contains the seeds of its own revolution.
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Because there's no one else to wag it for him.
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It doesn't matter, they don't have any electricity any more.
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One. He gives it to six Californians, thereby reducing the problem to an earlier joke...
In earlier work, Wiener [1] has shown that one mathematician can change a light bulb.
If k mathematicians can change a light bulb, and if one more simply watches them do it, then k+1 mathematicians will have changed the light bulb.
Therefore, by induction, for all n in the positive integers, n mathematicians can change a light bulb.
Bibliography:
[1] Wiener, Matthew P., <11485@ucbvax>, Re: YALBJ, 1986
or