The number of quotes you need to know really depends what book it is, and I think you're going about it the wrong way as it seems like you're just re-reading the book and picking out quotes which look pretty. Well, quotes are only as good as the situations you can use them in, and these situations would happen to be your ideas or arguments. These will tend to revolve around a theme, so first, identify and understand the themes of the book, and if you're struggling, this is one of the situations where study guides CAN actually help. Next, this is now where you start quote hunting in key scenes of the book for quotes which support your ideas relating to these themes. Perhaps later, you can do another full re-read once you've drained out the majority of the key quotes to find the smaller ones, but for now, stick with this until you've got some sort of foundation to start your essays with.