… can be found in the Stellungnahme und Gedanken zum Generalplan Ost des Reichsführers SS ("Opinion and Ideas Regarding the General Plan for the East of the Reichsführer-SS"), dated April 27, 1942 and written by Dr. Erhard Wetzel, the director of the Central Advisory Office on Questions of Racial Policy at the National Socialist Party, which is mentioned here and here.
Readers may ask what Wetzel’s mention of a "5 to 6 million" order of magnitude is supposed to mean.
That would be a good question, and the answer would be: it means that, based on what was known to him, Wetzel assumed there were about 5 to 6 million Jews in the area he was referring to at the time of his writing.
Now ask this question to "Revisionists". In their conspiracy theories, mentions of the number "6 million" prior to the end of World War II apparently mean something more.