According to Amazon’s CEO Jeff Bezos, “millions of people now own Kindles.” Sadly, Amazon has always kept the exact number of Kindle sales under wraps. According to some analysts, consumers in the US bought roughly 3 million e-readers in 2009 and the majority of these were probably Kindles. Amazon also doesn’t release any data about how many e-books it sells. In a press release that accompanied the company’s Q4 earnings report, however, Amazon yesterday announced that it now sells six Kindle books for every 10 physical books when both editions are available.

Given that Amazon doesn’t give give us exact numbers, we have to take the company’s word at face value. The idea that Amazon is getting close to selling just as many e-books as physical books when both editions are available still seems hard to believe however.

Explaining These Numbers

One explanation for the phenomenon comes from Slate’s Marion Maneker. Maneker points out that books that are available on the Kindle tend to be bestsellers and Amazon’s strategy of pricing new releases at $9.99 means that Amazon is probably selling most of its Kindles to users who are also the heaviest consumers of these commercial titles. After all, even though Amazon already offers steep discounts on recent hardcover releases, the Kindle edition is always the cheapest option. Indeed, those publishers and authors who want make use of Amazon’s new 70% royalty option have no other choice but to sell their Kindle editions for at least 20% below the price of the physical book. If Kindle owners are also Amazon’s best customers and also the most likely to buy bestsellers, then these numbers make sense.

What About Sony and Apple?

Just like Amazon, Sony doesn’t release exact numbers of its e-reader sales, though in an interview with TechOn earlier today, Fujio Noguchi, the deputy president of Sony’s Digital Reading Business Division, notes that Sony’s e-reader sales in the US quadrupled on a year-over-year basis.

Right now, of course, all eyes are on Apple’s new iPad, and even though it doesn’t feature an e-paper display, the iPad does have the opportunity to capture a large share of the e-reader market. According to Steve Jobs, Apple expects to sell e-books for the same price as Amazon, though it isn’t clear if he expects Amazon to raise its prices or if Apple plans to sell new releases at $9.99.