Thinking about self-publishing? You're not the only one! Publishers Weekly reports that there were more than 750,000 self-published/micro publisher titles in 2009--while the number of traditionally published titles slipped to just over 288,000 `(the story isn't specific but I assume these are just US figures).
Among the traditionally published books, fiction made up 45,000 titles. In non-fiction, the categories that were up in number were children's books, education, technology, science, personal finance, biography, and religion.
In the titles from self-publishers or micro-niche publishers, many were reprints of books in the public domain or print on demand titles. Bibliobazaar alone accounted for more than 270,000 titles. Never heard of Bibliobazaar? Me neither. Nor have I heard of Books LLC (more than 220,000 titles) or Kessinger Publishing (190,000 titles).
Among the better known self-publishing entitites, Createspace (which is a subsidiary of Amazon) put out almost 22,000 titles while Lulu accounted for 10,386.
The report didn't mention the average sale of self-published books, but I've read other sources that indicate it's under 20 copies. Of course there are also titles at the other extreme--self-published books that sell extremely well and often lead to a massive deal with a traditional publisher as a result of this success.
(Whether you're writing to self-publish or to attract a traditional publisher, you'll find lots of useful information in my book, "Your Writing Coach." You can get it from Amazon or your other favorite online or offline bookseller.)