The number of international students learning mathematics is much greater than that of Vietnamese students. You use "is" instead of "are" because the subject is "number", not "international students". "The number * learning mathematics is much greater" I also used "greater" than "more" because this is the form more used. You could say: "There are many more international students learning mathematics than Vietnamese." When we compare "number" of something verses "number" of something else, we use "greater than".