The numbers below are based on MY experiences... I'm sure you can find other authors whose numbers are different from these...but the following s/b pretty darn close. The whole idea is to give you an approximate number of books you would need to hit the OVERALL Kindle List only.
The rankings are interesting to look at if you have a book out there and you are hoping to, for example as of January 2012, get on the Top 100 Romantic Suspense Bestsellers List on Amazon. If I look at the Romantic Suspense Kindle Ebook List and click on the book title of #99 or #100 and that book has a Bestsellers Rank of 3,865 (#99) and 3,875 (#100), then I know I need a 3,875 or better to get on that list. As of 1/7/12 Finding Kate Huntley is #18 with an overall ranking of 446 and selling over 125 books a day. ABDUCTED is #40 with an overall ranking of 1,087 and selling over 80 books a day. And Dead Weight is #69 with an overall ranking of 2,182 and selling about 65 books a day.
Amazon Bestsellers Rank is the number you find beneath the Product Description. Every book on Amazon has an Amazon Bestsellers Rank. Click on any title and then scroll down until you see it.
March 2013 update: rankings have changed substantially in the past few months and I am making changes to reflect rankings and book sales as information is given to me.
Amazon Best Seller Rank 50,000 to 100,000 - selling close to 1 book a day.
Amazon Best Seller Rank 10,000 to 50,000 - selling 3 to 15 books a day.
Amazon Best Seller Rank 4,500 to 10,000 - selling 15 to 25 books a day.
Amazon Best Seller Rank 3,000 to 4,500 - selling 25 to 50 books a day.
Amazon Best Seller Rank 500 to 3,000 - selling 50 to 200 books a day.
Amazon Best Seller Rank 350 to 500 - selling 200 to 300 books a day.
Amazon Best Seller Rank 100 to 350 - selling 300 to 500 books a day.
Amazon Best Seller Rank 35 to 100 - selling 500 to 1,000 books a day.
Amazon Best Seller Rank 10 to 35 - selling 1,000 to 2,000 books a day.
Amazon Best Seller Rank of 5 to 10 - selling 2,000 to 4,000 books a day.
Amazon Best Seller Rank of 1 to 5 - selling 4,000+ books a day.
That's all I've got for now. If you have more information, please share. Thanks.