Asking authors about sales figures is like asking some people about their weight or shoe size. It's very personal and potentially embarrassing information, and writers are often reluctant to give it out, even to other writers.
Success (in the commercial sense--not talking about personal expectations here) depends a lot on context and publisher expectation. If your publisher prints 50,000 copies and your book only sells 20,000, you might be in trouble when it comes to persuading the publisher to take you on for another book. If, on the other hand, the publisher printed 10,000 and then had to go back to press for several thousand more, the publisher might be pleased enough to give you a nice deal on a followup book, because you exceeded expectations. A new author published for the first time in mass market paperback might be considered to have done pretty well with sales of 20,000 (assuming a decent sell-through), while for an established author whose sales have been dropping over a few books, that same 20,000 might be disastrous.
The above is highly simplified, but you can see how much "success" depends on context.
I'll bare my soul here and reveal that my last mass market paperback ("The Garden of the Stone") sold around 15,000 copies, and had a sell-through of 70%. 15,000 isn't a huge number, but the sell-through is quite a bit better than average for sf/fantasy.
- Victoria
P.S. I guess I should define sell-through--it's the ratio of books sold to books printed. So a book that had a print run of 20,000 and sold 10,000 would have a sell-through of 50%.