The number of "unique individuals (who) served at least 1 day in the active component" from from October 1, 2001 through October 1, 2016, according to an article in the January 2017 issue of the Armed Forces Health Surveillance Branch's Medical Surveillance Monthly Report -- Selected demographic and service characteristics of the U.S. Armed Forces, active and reserve components, 2001, 2009, and 2016.
An additional 2,561,073 served at last one day in the reserve component.
The researchers used three specific time points to gather demographic surveillance data -- 1 October 2001 (the beginning of Operation Enduring Freedom); 1 October 2016 (15 years later); and 1 April 2009 (the midpoint of the entire time period). Some data points from the report:
The report also provides data on age, ethnicity, rank/grade, service branch, and percentages cared for in a military treatment facility or outsourced TRICARE facility.