Too Many? Twists and extra story lines in books help make the reader more interested in what's going on. These also give more content to the book in general. A book with many layers to the story might be more exciting and suspenseful, but what if the layers are laying just a little too thick? There needs to be a focus to a book, and if there are a dozen different hoops that the character has to jump through, or too many stops along the way, the focus becomes fuzzy.
Too Few? But what if there isn't enough to the story? If the story is too straight forward the book might seem to simple, and easy to predict. There's nothing keeping you from finding the end, so it comes much quicker than it might if there were a few more stops along the way. The easy answer is that the story is just plain boring.
Just Right? Can there be a 'Just Right' amount? There isn't a number of twists a story has to have to be enough, or too many, it really depends on how well they are executed. Can the author keep the goal in mind? Can they keep it interesting? Can they keep it manageable?
Writing a good story can be one of the more difficult tasks that people may attempt because there is so much to think about when crafting a story. Let me know if you have thoughts on this subject!