Prior to WW2 the communists killed millions, for all kinds of reasons, mostly ideological or racial. They were every bit as vile and evil as the Nazi's but as socialists and one of the 'winners' they have been allowed to get away with it, to a degree.
You can mess about with the death tolls in the Nazi camps, but the evidence that millions died is overwhelming. There were just 4 purpose built extermination camps and 2, including Auschwitz that were combined work/death camps.
The number of deaths in those camps were around 3-4 million. The entire Nazi machine killed far more, in work camps, summary executions and myriads of other 'camps'. I do not think that 12 million overall is an unreasonable number.
The death toll in the Soviet Union in the same period were equally huge, the Holomodor alone accounted for a similar number of deaths as the 6 Nazi death camps combined.