That is just the estimated number of people killed specifically in the death camps, the overall total you have been taught is given on the main page for the Holocaust.

You likely triggered an automod filter in /r/history for catching Holocaust deniers, as you are a throwaway account, and holocaust deniers are very active on reddit.

3 million died in the death camps, but millions of others died simply from summary execution like at babi yar or from starvation in the ghettos.