Learn Spanish (Latin America): ¿Cuántos libros tienen ustedes?

The question word cuánto changes to match the noun it describes. Since libros (books) is masculine and plural, you use the masculine plural form cuántos. If you were asking about apples (manzanas), you would use cuántas.

Why do we use tienen for "you have"? Isn't that the form for "they"?

In Latin America, ustedes is the standard way to say "you" to a group of people (like "you all" or "you guys"). It always takes the same verb form as "they" (ellos or ellas). So, tienen means "you all have" when paired with ustedes.

Why is ustedes placed at the very end of the sentence?

In Spanish questions, the subject pronoun usually comes after the verb. While a normal statement would be ustedes tienen (you have), a question flips the order to tienen ustedes (do you have).

Do I always need to include ustedes when asking this question?

No, you can leave it out and just ask ¿Cuántos libros tienen? if it is already clear who you are talking to. However, because tienen can also mean "they have," including ustedes makes it perfectly clear that you are asking "you all."