I remember seeing an article about this, written by a book reviewer, a year or so ago. That book reviewer talked about the many books they were reading at once -- four or five at a time, as I remember.
I'm not quite that bad, but I do often have a couple of books I am reading at any one time. I usually try to stick with just one novel, though -- the other books I am reading are often nonfiction books, and at least one is typically history. For example, recently I've been researching the Roaring Twenties, so one of the books I am currently reading is Last Call, a history of Prohibition in the United States by Daniel Okrent, and another is a short history of Chicago. These are both in addition to the novel I am currently finishing up (Under Heaven by Guy Gavriel Kay). I also often have an ebook in progress that I am reading in Barnes & Noble stores on my Nook (I love their Read In-Store feature!), and sometimes I have an issue of a magazine that I am working my way through.
Usually, though, I'm never reading more than three books at any one time. I find that if I read too many, or if I take a break on one and come back to it later, I feel rather lost and it takes me a little while to get back on track.
What about you? How many books do you read at once, and what genres do they usually fall into? What books are you reading right now?