Based on the data from "Winning Middle School: How Many Students in 9th Grade?", the number of students in the 9th grade across the United States is approximately 5.5 million. This figure represents a significant portion of the total student population in middle schools, as the 9th grade is the transitional year from middle school to high school.

The reason for the substantial number of 9th graders is multifaceted. Firstly, the U.S. education system typically enrolls students in 9th grade at the age of 14, which is the age at which many students complete middle school. Secondly, the U.S. has a large population, with a significant number of students entering the educational system each year. According to the National Center for Education Statistics (NCES), the total number of public school students in the U.S. was approximately 50.7 million in the 2019-2020 school year. This includes students from grades K-12, with a substantial number of them being in the 9th grade.

Additionally, the demographic trends in the U.S. contribute to the high number of 9th graders. The U.S. has a relatively young population, with a large cohort of children born in the early 2000s reaching the age of enrollment in 9th grade. This demographic bulge, often referred to as the "Millennial Generation," has led to an increase in the number of students in the 9th grade.

Furthermore, the educational policies and practices in the U.S. also play a role in the number of 9th graders. Many states have adopted policies that encourage students to remain in school longer, which includes completing the 9th grade. This is in line with the goal of increasing the high school graduation rate and ensuring that students are better prepared for higher education and the workforce.

In conclusion, the number of students in the 9th grade in the United States is approximately 5.5 million, driven by demographic trends, educational policies, and the large population of the country. This figure reflects the importance of the 9th grade as a critical transition year in the educational journey of students.