According to the November figure, there has been 200,226,838 NICS check performed since the system was implemented. It should be carefully taken since the number of check doesn't show the exact number of guns sold.
PDF: http://www.fbi.gov/about-us/cjis/nics/reports/total-nics-checks-nov.-30-1998-november-30-2014
Article: http://www.guns.com/2014/12/09/nics-background-checks-break-200-million/
From: Guns.com