There's no definitive answer for how many e-books a Kindle can hold, as there are several different models to choose from, and no fixed size for the e-books loaded onto them, but it is possible to estimate a figure. As of April 2013, there are three Kindle e-readers and three Kindle tablets on sale in the UK, each capable of storing and displaying e-books.
The basic level Kindle e-reader offers a 6-inch e-ink display and comes with 2GB of internal storage (of which around 1.25GB is available for user content). Amazon's official description for the product suggests up to 1,400 e-books can be held on the Kindle, which would put the average e-book size at around 868KB.
Kindle Paperwhite and Paperwhite 3G
The Wi-Fi Kindle Paperwhite and the 3G-enabled model both have a 6-inch display offering a higher resolution and superior contrast to the basic Kindle. As with the cheaper Kindle model, 2GB of storage is included on board, of which 1.25GB is free for user content. However, Amazon states up to 1,100 e-books can be held on these devices, suggesting a slightly higher average e-book size of 1.14MB.
The Kindle Fire tablet has a 7-inch multi-touch colour display running at a resolution of 1,024 by 600 pixels. Of its 8GB of storage, 5.5GB are available for user content. Taking the average estimated e-book size inferred by Amazon for both the Kindle and the Kindle Paperwhite (a little over 1MB), this would hold 5,478 e-books if no other content (such as apps or movies) were loaded onto the device.
Kindle Fire HD
The 7-inch version of the Kindle Fire HD tablet has a display resolution of 1,280 by 800 pixels. Two models are available with either 16GB of storage (around 12.6GB free) or 32GB of storage (around 26.9GB free). Using Amazon's estimates of the average e-book size, you could fit approximately 12,550 books on the smaller model and 26,793 books on the larger one with no other content installed.
Kindle Fire HD 8.9 Inch
The bigger Kindle Fire HD tablet comes with a 8.9-inch screen and a resolution of 1,920 by 1,200 pixels. As with the 7-inch device, 16GB and 32GB models are available, but the space available for user content is slightly higher -- 12.7GB and 27.1GB respectively. This puts the approximate e-book capacity at 12,649 for the smaller model and 26,992 for the larger model with no other content installed.