Two — one to screw it most of the way in and the other to give it a surprise twist at the end.
or
Because sometimes you can get rid of cancer.
or
he said "Not you again".
or
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
if you mess with the United States, there will be hell toupee."
or