Is there a general rule of thumb for how many subplots should be in a novel? How many are too few? Too many?
You want enough to keep your story interesting, but not so many that the reader can't remember what's going on with who. How's that for a definitive answer?
I'm gonna' go out on a limb and really commit myself here and say between two and ten. Part of it depends on how complex your main plot is; how complex the subplots; whether the subplots are needed to move the story forward (good), or if their main function is to add pages (bad); if the subplots involved the main characters or side characters; etc. etc.
Here's my general rule--if I'm bored, it needs more complexity, which can be provided by subplots. If I have to read with a pencil and paper to keep all the characters and plots straight, then you've got too many.