There's no single definitive answer to how many women's colleges there are in the US. The number fluctuates depending on how you define "women's college." Some institutions may be single-sex but not exclusively for women (e.g., they might have some male students in graduate programs). Others might be historically women's colleges that now admit men.
There are approximately 40-50 institutions that are currently either exclusively women's colleges or historically women's colleges that still primarily serve women. However, this number can vary based on how you classify institutions that have recently changed their admissions policies.