31 million Americans have attended some college but failed to earn degrees.

http://www.insidehighered.com/news/2014/07/29/clearinghouse-report-identifies-31-million-who-attended-college-didnt-complete#sthash.jHkqhXa1.dpbs

So says the National Student Clearinghouse Research Center:
http://nscresearchcenter.org/category/reports/signature-report/