I read somewhere that there are seven plots. I researched this on the internet and found a page with lots of different lists of numbers of plots.
1 plot. Lots of people like this, but obviously this is just a structure that they’re describing, not a plot.
3 plots. Foster-Harris proposes this. It sounds kind of interesting and warrants further study.
7 plots. Jessamyn West proposes man against various entities—nature, machine, god, self, etc. This doesn’t cast much light on opera plots.
20 plots. Ronald Tobias. This is actually a useful list.
However, opera is a special case. For opera I would like to propose 3 plots:
1. love problems that resolve badly.
2. love problems that come out well (practically every comic opera).
3. everything else. The list of twenty can be used to break this out.
My number one includes Carmen, Butterfly, Orfeo, Otello, Masked Ball, La Traviata, La Boheme, lots of stuff.
Turandot is a riddle opera.
Fidelio is rescue.
Rigoletto is about revenge.
Everything else is love. Tip for composers—find a story about love and make a libretto out of it. John Adams' topics aren’t operas. Vanessa is about love and is gaining in popularity with the passage of time.
I am going out on a limb here—maybe this is the reason for the failure of modern opera. Opera is about love.