This is the 10631st most frequent Spanish word.
cuántos
"Cuántos" means "how many" in English.
The word 'cuántos' is used here as an interrogative pronoun to inquire about the number of years, relating to age.
¿Cuántos años tienes?
How old are you?
The word 'cuántos' serves here to question the estimated quantity of books in the library.
No sé cuántos libros hay en la biblioteca.
I don't know how many books there are in the library.
In this example, 'cuántos' is applied to express wonder or evaluation of the quantity of friends in attendance.