LOL OK, First question is honestly, it depends. I have well over 1,000 books on mine and plenty of space to keep adding. I have the basic Kindle with less than 2 gigs of space and it's only 1/4 full with that. So, you can put a ton on these devices.
The second question is a little more complex. You can use the cloud drive, as stated, or get something called Kindle for PC (it's free from Amazon). You can read them either way. I back up my kindle content on a flash drive regularly. If the books are bought from like Amazon, they aren't going to open with something like Calibre as they are protected content. If you put something like fanfiction on it, you can open it with anything, provided it supports that content.
This is Calibre
calibre-ebook.com/ You can not only use it to read on the PC, provided the books aren't protected, but can upload anything you want that way. You can also get the fanfic downloader plugin for it and it'll put the fics right on your kindle (it's one of the few that supports Dokuga and almost every site out there except AFF). It's what I use to transfer things around and put it on there. Trust me, it's easy to run. Even for some of us more tech-challenged!