Because Amazon is very closed about sales information, it’s difficult to really judge what’s going on behind their algorithms. Publisher’s Weekly recently studied a few titles and determined that it takes about 300 sales of a single title per day to hit that marker.
Note, however, that these numbers are for the primary categories. The rankings also placed the titles in the top 10, not just the top 100. If you set your book into a smaller category, you need far fewer sales to hit the top 100. You’ll still need hundreds of sales per day to get into the top 10 but in the top 100, you’ll receive the additional benefits of Amazon’s algorithms.