There's no single, publicly available, perfectly accurate count of the number of high school sophomores in the US. Data on student enrollment is collected in different ways by various organizations (like the National Center for Education Statistics), and it's not always broken down precisely by grade level. Furthermore, enrollment numbers fluctuate from year to year.
To get an estimate, you would need to look at overall high school enrollment figures and then attempt to proportionally estimate the sophomore class size, knowing that sophomores represent roughly one-fourth of the total high school population (assuming a roughly equal distribution across four grades). Even then, this would only be an approximation.