What does cuantos libros son mean?

¿Cuántos libros son?

How many books are there?

Literally asking "How many books are?" The qualifier depends on who is asking whom the question where. You are holding a stack of books and some one asks this question, they are asking how many books you have. You are standing in a library and some one asks you that question while gesturing to the library, they are asking how many books are in the library.