How many X does it take to change/screw in a light bulb? N, Y.
Pick any subject for X. Then Y has to contain some common knowledge (a prejudice, description, image or idea) about that subject. Although N ('Number') comes first in the second sentence, it generally should be the quantification of that primary characterization, in that it makes sense because of Y. (I doubt whether or not this last description is really helping.)
This formula is often being used to target an individual or a group its stupidity - after all, how hard is it to change or screw in a light bulb? So when thinking about filling in your ideas, it might help to think of some flaws or stereotypical habits of your subject. This formula can also be used to highlight more positive characteristics, but in that case it may be harder to find it humorous because of this whole light bulb situation.
Here are two examples and a link with a very large collection of examples of this formula:
How many Marxists does it take to screw in a light bulb?
None, the light bulb contains the seeds of its own revolution.
How many voyeurs does it take to change a light bulb?
Only one, but they would much rather watch someone else do it.