Publishers Weekly has an article that estimates the number of sales required to reach a Top 5 ranking on Amazon. Something doesn’t look right. PW estimate that as few as 300 print sales per day will win a coveted spot in the Top 5. This is based on monitoring the ranking and sales for one book for two weeks.
Here’s why I think the estimate is off. My best sales day was 660 books for Tempest at Dawn. This occurred after a television appearance and sales remained above 300 per day for nearly a week. Although the book made #1 in several categories like Historical Fiction, it only briefly broke through the top 100 for general sales. Also, The Shopkeeper has had spurts of very good sales, but only temporarily has broken the Top 1,000 in books. Sales would need to increase exponentially to get a Top 5 ranking.
Is it possible that the shift to e-books has so dramatically reduced print sales at Amazon that 300 sales per day would land you on top of the heap? I doubt it. It’s more likely that faulty assumptions resulted in the smallish number. We’ll never know, of course, because Amazon keeps all of its sales data close to the vest.