American universities have application systems and flexible enrollment policies, allowing students to apply across disciplines. However, it will be more advantageous if you have a theoretical basis for applying for a major before applying. For applying for a master's degree in mathematics, the best applicants are students majoring in mathematics, followed by those majoring in mathematics, such as statistics, information and computing science, and finally students majoring in science, such as biology, physics, computer and so on. The most difficult category is liberal arts majors (English, history, art, etc. ) were promoted to master's degree in mathematics. Some schools in official website will clearly list the requirements of prerequisite courses, and most schools will require at least three basic mathematics courses: Calculus, Linear Algebra and Probability Theory. I hope it will be helpful for your application.