One. He gives it to six Californians, thereby reducing the problem to an earlier joke...
In earlier work, Wiener  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.
 Wiener, Matthew P., <11485@ucbvax>, Re: YALBJ, 1986
In their Sleevies!