It really depends on the period they use to calculate it. My book was very briefly the #1 programming book on Amazon[1], which is, I think, a decent-sized category. I don't know the exact numbers, but I probably sold a few hundred copies that day. But, more importantly, I think I had a narrow spike of a lot of sales in a couple of hours right when the book first launched.
[1]: http://journal.stuffwithstuff.com/2014/11/20/how-my-book-lau...