That comment uses the figure quoted by the previous comment. But look at the pdf linked there for the UN report - that's not a number of rapes, that's a number of "crimes recorded in criminal police statistics!" No wonder it's much lower than the real figure. (I don't even know if it includes all reports/accusations or just counts found guilty by a court.)

Incidentally that document is missing some of the more interesting statistics for the US, while it has them for other countries. "Rape average prison sentence served" is one.