I don't limit the book, but we (or rather, I) also don't read them all. Before we leave the library we go through what we have picked out just to make sure we need each book. When we bring the books home the kids know that after reading the books they need to be put back in our designated library bag, this really cuts down on misplaced books.
We have two times a day when I always read aloud to them. I will read whatever books they ask during the alloyed time, we may not finish them though. I usually read a picture book when they ask, but if I am busy or we have something else to do, I will decline. My kids would love it if I read to them all day, but it isn't realistic and they have slowly learned that. Sometimes the older two will read to the smaller two, which helps a lot.