I find that it's so much easier to look for the next book with Calibre than on my Kobo. And since it only takes me seconds (ok, say a minute) to connect the reader and transfer the book, I could probably get away with having only one book on the reader.
That said, I have a couple of hundreds on it (almost all non-fiction). I tend to read maybe 4-5 books at a time (mostly non-fiction, usually 1 fiction at a time).
If I travel and won't have access to my library, I usually add a bunch a books just in case.