Amazon estimates that an eBook runs around 2KB/page. If you are reading a 300 page book, you can expect it to be around 600 - 800 KB.
There are 1024 KB in a MB, and 1024 MB in a GB. Given that some of that space is taken up by the OS and apps, let's estimate around 6 GB of space available on the Kindle.
With an estimate of 6GB of space available, you should easily be able to fit over 7,000 eBooks on the device.