"There are many books."
Translation:Multi libri sunt.
If I'm understanding Latin correctly, "libro" would be ablative singular. "Books" in this sentence is plural & is the subject, which means it would need to be in nominative plural, which is "libri". "Libros" is accusative plural, which is used when "books" (pl) is the direct object (i.e., something is being done to it, as in "I read books").