How many books do you carry on your reader?
All of them. At least, on my JE100, I need to convert most of them still to read on my Mini.
Do you have an sd card slot?
Yep, both on them. I have my books on the MicroSD on my JE100 and I need a new SD card for my Mini.
If you are fortunate enough to have folders, how do you sort them?
The JE100 uses Mobipocket Reader, so I do have my files in a folder structure, even though the application ignores them. The Mini does show the folders and I use the same folder structure:
Author - Book
Author - Series - Book
Please post on why you carry the books you do.
I like to have a choice. When I finish a book, I want to be able to pick any book I have without having to search for it on my PC. Everything at one place, love it!
Or anything else you'd like to comment on. 
I need to convert more books, so I can read them on my Mini! I'm reading a different book on my Mini as on my JE100. I've no idea how many books I have...