I love them...possibly too much. More than once an intrepid critique partner has told me,"it's too complicated, there's too much going on."
As I've blogged before, I'm a plotter. I detail the main plot, primary characters, external conflicts, and the red herrings. The subplots, however, those sneaky bastards work their way into the story as I'm writing the first draft. Why? Because the subplots are often birthed by my deeper understanding of the antagonists and secondary characters.
Yes, you read that correctly. Subplots, for me, are results of the Goals, Motivations, and Conflicts of the secondary characters -- be they part of the Scooby Gang or the Anti-Justice League. The subplots cross with the main plot because they impact the protagonist's External Conflict or Internal Conflict in some way.
Usually, those impacts come as surprises to me.
Some writers who are Pantsers resist plotting because it takes away the fun of discovery. For me, subplots are where the creative revelations burst forth. They are what causes the final chapters to end in a very different way from what I had outlined. (Yes, yes, shocking that the actual written story doesn't stick to the initial outline. ~cough~ Not. ~cough~)
During the first edit I go back and re-lace the subplots. Some of the threads are woven together (villains in collusion!), some are more tightly tied to the main plot (lust isn't love unless it's twue wuv!), and some are unraveled completely (exorcise those superfluous characters!).
How many subplots are too many? Uhm. When the story gets too complicated or confusing? Really, I'm still working on striking that balance. I'm fairly certain there is no fixed amount. It all depends on the story you're telling...and the cap on the word-count.