Nice one, Doorstop. We don't get many mathematical jokes. I spent a whole year reading the subject at uni and didn't hear one of my fellows crack a joke in all that time.
Q. How many mathematicians does it take to change a lightbulb?
A. None. It's left to the reader as an exercise.
Alternatively...
Q. How many lead singers does it take to change a lightbulb?
A. 20. One to hold the bulb and 19 to drink until the room spins.
Let us return to our mathematicians:
Q. How many mathematicians does it take to change a lightbulb?
A. One. He gives it to the lead singers thereby reducing the problem to the previous joke.
If k mathematicians can change a lightbulb and one supervises, then k+1 mathematicians will have changed the lightbulb.
We have already proved that one mathematician can change a lightbulb so by induction, for any positive integer n, n mathematicians can change a lightbulb.