Before the CMP took over the program in '96 it was run under the DCM. Civilian sales date back to the mid 1950's.
The figure would easily run into the millions, but you'd be hard pressed to get an accurate figure. But why limit your guess to M1 rifles & carbines... don't forget the 1903 rifles, Krags, 1917's, rimfire rifles, etc, etc. Do drill rifles and receivers count too?