There's no single answer to the average number of students per US college because it varies dramatically depending on the type of institution. Large public universities can have tens of thousands of students, while small private liberal arts colleges might only have a few hundred.
To get a meaningful average, you'd need to specify the type of institution (public, private, 2-year, 4-year) and possibly even further refine it by size or type of program. Any overall average you find would be highly misleading.