As for how many plots there are, that depends on how broadly you define "plot." If you go really broad, there are only three: man vs. man, man vs. nature, man vs. himself. I tend to see plot as slightly more detailed than that…

Michael: That is a vulgar version of what Aristotle said in the Poetics, which is that dramas require a beginning, middle, and end. He did not discuss "acts" and in fact Greek plays were not written in acts at all.

Jim: Who knows? In the meantime I have to concern myself with mundane tasks like making a living.

Steve: I’d like to help out here, but I’ve never read Confederacy of Dunces, though, or perhaps because, it has often been recommended to me, and I haven’t looked at The Good Soldier for nearly twenty years. You tell me.

There’s a French book published in 1916 as "The Thirty-Six Dramatic Situations" by Georges Polti and described further here:
http://www.darkshire.org/~jhkim/rpg/theory/plot/the36plots.html

But, I concede it may be profitable to provide a list the books that can or should be written.

By the way, Aaron, can we expect any such titles from the pen of Mr. Haspel…?

Act I: We meet the protagonist in his nature, primeval state; we learn who he is before the events of the story change him.

Plot Point I: Something happens which sets the main story in motion by giving the protagonist one primary goal (i.e., Meg Ryan hears Tom Hanks on the radio in Sleepless in Seattle and decides she must make him love her).

Act II: The protagonist pursues his goal, facing and overcoming greater and greater obstacles along the way.

Plot Point II: The Act II climax, which resolves the great sweep of story that comprises Act II.

Act III: We see the impact of the protagonist’s Act II adventure on his life, his character, the other people in his life, etc.

For the most part, at least in movies, Acts I and III are short –15 minutes or so. Act II is long: around an hour.

The mistake most critics of screenwriting theory, workshops, etc., make is in thinking that preaching this sort of structure makes movies simplistic, boring, predictable, etc. As Aaron points out, this is a red herring. When you get used to thinking in these terms, you realize that practically every movie, novel or play you can think of fits fairly neatly into this paradigm: the great work right along with the mediocre. My understanding, Aaron, albeit mostly from secondhand sources, is that 3-act structure derives ultimately from Aristotle. True?