People are often surprised when I say that I have lots of writing projects finished, some that I've never even tried to sell. I guess they assume that an author has one idea at a time, works it out, and then maybe sits down to figure out another. Of course lots of authors, even great ones, only had one or two. As Jimmy Durante used to say (but not about plots) I've got a million of 'em, in many stages of development at any given time.
There are lots of crazies like me in the mystery field. From what I can tell, a lot of us have new plot ideas all the time. A news report, rumor, or juicy tabloid story can get us thinking, "What if..." Often a plot is sketched out, sometimes a few chapters or even a full story written down. It may not be good. It may not ever get polished. But we've always got more than one idea.
Maybe it's genre fiction in general, which requires lots of entries for eager fans but has fads that may make a story that was hot yesterday unsellable today. Maybe it's because crime is ever-fascinating, since man's inhumanity to man is so obvious and yet so unbelievable. Maybe it's just that we like coming up with a story, answering our own inner voice that says, "Wow, that's cool." But for many of us, one story, even one sub-genre, just isn't enough.
You'll be told that you must settle into a niche and write what you're good at. Maybe, but you probably won't know what you're good at until you try a few things. While I agree that a writer's quality may suffer from trying to do six projects at once, I find that when the time is right, I zero in on the one that's most important to me and pull it together.
So do too many plots spoil an author? I say no, as long as you are always evaluating yourself and the market. Maybe the piece that excites you today will get put in a drawer tomorrow, but six months from now, it may be just the thing to finish, polish, and show to the world.