I usually keep a small collection of fiction and non-fiction books on my kindle, plus several anthologies of short stories or non-fiction essays for when I don't feel like starting a new book or just want to take a short break from one. Also there are books I feel like I should
read that I keep on the kindle to make me feel guilty when I see it.
Of course, the number gets bigger over time . Sometimes I load a book intending to read it next but then something else catches my eye.
If the number of books is too big it makes it harder to navigate, so every now and then I delete everything and start over.
The only collections I have are "Nightstand" for books I am reading or expect to read soon and "Browse" for books that are good for reading a little at a time like anthologies or collections of articles and essays.