[WiseWeasel]

From Wikipedia's entry (http://en.wikipedia.org/wiki/Gulag):

"... during the period of 1928–53, about 14 million prisoners passed through the system of GULAG labour camps and 4-5 passed through the labour colonies."

Original source: Robert Conquest in 'Victims of Stalinism: A Comment.' Europe-Asia Studies, Vol. 49, No. 7 (Nov., 1997), pp. 1317-1319 states: "We are all inclined to accept the Zemskov totals (even if not as complete) with their 14 million intake to Gulag 'camps' alone, to which must be added 4-5 million going to Gulag 'colonies', to say nothing of the 3.5 million already in, or sent to, 'labor settlements'."

Subtracting the ~1.5M foreign prisoners of war, the 12M estimate seemed like a reasonable minimum number of Russians sent to the Gulag.