There's no single, readily available, precise number for how many minors are in America. The Census Bureau collects data on age, but it's not a constantly updated, real-time count. Furthermore, the definition of "minor" can vary slightly depending on the context (e.g., driving age, age of consent, etc.).
To get an estimate, you would need to look at the Census Bureau's data on population by age and calculate the number of individuals under the age of 18. Keep in mind that this would be an approximation, and the number fluctuates constantly due to births and aging.