There's no single SAT Math score that guarantees admission to Northwestern University. Admission is holistic, considering many factors beyond just test scores. While a high score is beneficial, Northwestern looks at the whole application, including GPA, extracurricular activities, essays, letters of recommendation, and the rigor of your high school coursework.
While Northwestern doesn't publish minimum scores, successful applicants generally have exceptionally high scores, often well above the 700 mark in the Math section. However, even a perfect score doesn't guarantee admission. Focus on a strong overall application rather than fixating on a specific number.