There's no definitive, publicly accessible database tracking the exact number of single-sex schools in the US. The number fluctuates as schools open and close, and different organizations may use varying criteria for inclusion (e.g., whether a school is entirely single-sex or offers single-sex classes within a co-educational setting).
Therefore, providing a precise number is impossible. To find information, you would need to conduct extensive research across various state education departments and private school associations.