I have as many as my Reader can take without using a memory card. Just because I can.
I delete books as I read them and I tag them in Calibre as "read--keep" ("bookshelf") or "read--not keep" ("box") to distinguish between books I may want to read again and books that I would donate if they were not electronic.
I use folders for my series to group them together and make them easy to find. The rest of the tags are the ones that come with the book.