As someone with a large archive and many devices I can tell you can download any book unlimited times. The book may have restrictions on how many devices at one time the same book can be on but there is no download limit on any book whatsoever.
From Amazon's Kindle Download Help Page:
Downloading Content to Multiple Kindle Devices
Content purchased from the Kindle Store can be downloaded to your Kindle device or free Kindle reading application as long as you've registered the device or app to the same Amazon.com account from which the Kindle content was purchased. There is no limit on the number of times a title can be downloaded to a registered device or application, but there may be limits on the number of devices or applications (usually six) that can simultaneously use a single book.
That means you can download and read your books on any Kindle device you own or Kindle reading application as long you've registered each device or application to the Amazon.com account where your Kindle Library is stored.