A friend recently confided in me that he is "cautious" about opening new books, because, he said "I hate the idea of not liking the book, and then feeling guilty if I don't finish it."
Although this might strike some readers as mildly neurotic, I think that a lot people actually feel this way. It might be the generally high cost of books nowadays, or simply a Calvinistic impulse to "finish what one has started."
I generally try to finish all the non-fiction books I open, because they will at least provide me with information. However, I occasionally abandon novels midway through (or earlier), if the story doesn't grab me.
How many? you might ask. I haven't kept records, but I would say about 1 out of 20.
I would recommend starting more books, even if you end up abandoning a certain percentage of them. Unless you are truly frivolous about it, the net affect will still be that you read more books.
If you are concerned about the cost of books, then focus on lower price ebooks, books purchased in secondhand venues like Half-Priced books, or that old standby, the public library.