I was 20 at the time and serving in the army. There I was actually leading company-strength motor marches.
Talk about overly worried parents.
As to your question, a quick Google arrives at a story claiming the number is 4,400:
"U.S. medical examiner and coroners' offices receive an estimated 4,400 unidentified human bodies every year, according to the first national census of medical-legal death investigations, "Medical Examiners and Coroners' Offices, 2004." Of these, about 1,000 are still unidentified after one year, and 600 are buried or cremated."
http://www.sixwise.com/newsletters/07/07/11/unidentified-hum...