It depends on a number of factors. For one, the size of the publishing company (500-1000 copies is considered poor for a large publishing house), the location (the economy in the U.S. has been declining and book sales reflect this), as well as other things.
Here are some stats for you:
Here’s the reality of the book industry: in 2004, 950,000 titles out of the 1.2 million tracked by Nielsen Bookscan sold fewer than 99 copies. Another 200,000 sold fewer than 1,000 copies. Only 25,000 sold more than 5,000 copies.
The average book in America sells about 500 copies” (Publishers Weekly, July 17, 2006). And average sales have since fallen much more. According to BookScan, which tracks most bookstore, online, and other retail sales of books, only 299 million books were sold in 2008 in the U.S. in all adult nonfiction categories combined. The average U.S. book is now selling less than 250 copies per year and less than 3,000 copies over its lifetime.
But I agree with others, don't let this stop you. There are always those who sell beyond their expectations. Stephanie Meyer's Twilight series, case in point.
So write for you and enjoy :)