Amazon does not provide sales figures. There were probably more than 0.5M Kindle 1's sold. On January 28, 2010 Amazon said it had sold "millions" of Kindles, most of them Kindle 2's. One
analyst estimated 3.3M by April 2010, so there were perhaps 3M Kindle 2's sold. Estimates for all of 2010 range from 5M to 8M, with 8M being more favored. By now there must be at least 10M Kindle 3's.
What is even less certain is how many Kindle 1's are still being actively used.