I have around 500 personal docs on my Amazon Cloud, nothing else so I thought I'd see what that shows as. I downloaded the Amazon Cloud app for iOS (denied it access to photos as I have no wish to load my photos into Amazon's cloud, or anything else apart from ebooks) and took a look.
In My Storage it shows I have 5GB + 5GB for My Send To Kindle Docs. I've apparently used 70MB
I'm on Prime so not sure if that makes a difference.
Edited to say the website access actually shows exactly what personal docs are uploaded as well and the actual number so my estimate of 500 is actually 470. The website link shows more than the app.