Every year, a huge number of young people enter college across the United States.
Literal
American whole-land [in-では] every-year enormous number's young-people [subject-が] college [to-に] enter.
米国全土 is 'all of the U.S.' — 全土 meaning 'the entire land.' 膨大な数 is 'an enormous number.' The では marks the geographic scope. 大学にはいる means 'to enter university/college.' A factual observation in written register.