Basketball is the second most popular sport in the United States, when counting amateur levels. According to the National Sporting Goods Association, over 26 million Americans regularly play basketball, which is more than any other team sport. In 2020, the US population was 331 million, meaning that 7.9% of the population plays basketball.
https://www.sfia.org/press/433_Over-26-Million-Americans-Play-Basketball
Age groups distribution:
Data Source for the table below - Statista
Level of interest in basketball in the United States as of May 2020, by age
About 56.4 million students are projected to attend elementary, middle, and high schools across the United States (source).
About 19.7 million students are projected to attend colleges and universities in fall 2020 (source).
Fast Facts: Back to school statistics (372)
Elementary school is a kindergarten through 5th grade (ages 5-10), middle school is grades 6-8 (ages 11-13), and high school is grades 9-12 (ages 14-18).
Secondary education (= high school) in the United States is the last seven years of statutory formal education grade 6 (age 11–12) through grade 12 (age 17–18). It occurs in two phases. The first is the ISCED lower secondary phase, a junior high school or middle school for students grade 6 (age 11–12) through grade 8 (age 13–14).
About 17,000 high schools in the United States have basketball teams. All 50 states conduct statewide tournaments annually.
We assume that there are about 8M basketball players in ages 5-18 years in the US = 30% of all amateur players. 18+ players are the major 82%, but they usually don't train in a club with a coach.
Of the 50.7 million public school students (source):