The number of Jews in the world was estimated at almost exactly the same before and after WW2 - no missing six million.
Speaking of "six million", that's a number of special numerological and superstitious significance to Jews residing in Eastern Europe. You can find newspapers dating back before 1900 (and many between 1900 and WW2) which claim that six million Jews were somehow or other killed.
Cremation takes 2-3 hours. With 4 crematoria working day and night, Auschwitz could have cremated around 70,000 people in four years, far short of the 4.1 million claimed during the "Iron Curtain" period and also far short of the high hundreds of thousands / low 1 million claimed today.
Speaking of 4.1 million, somehow when that number was revised down by about 3 million the total number of deaths remained constant at 6 million.
Treblinka supposedly buried some 700,000 victims, forced a small handful of prisoners to dig them back up, cremated the remains in open air and then hid the whole operation with zero physical evidence.
The main source of information about Treblinka, a man named Jankiel Wiernik, reporting being shot in the shoulder but suffering no injury despite the bullet passing through his clothing and striking his flesh.
Hundreds of prisoners report being saved from an active gas chamber by Josef Mengele, including during time periods where Mengele was known to be elsewhere.
The whole story is full of these absurdities and incredibly flimsy if you start digging into it.
What is not flimsy is that there really was housing built to separate "undesirables" from the ordinary population, and as the war dragged on and German supply lines collapsed (a deliberate aim of the Allies) the Germans were no longer able to keep these people free of disease and well-nourished.
This led to the famous photographs of emaciated bodies and emaciated prisoners. However, those photographs were taken by the Americans at Western camps, where modern historians do not contend there was any extermination program at all. Nordhausen, for example, is often cited as photographic proof of the Holocaust (eg by Katie Couric interviewing Mahmoud Ahmadinejad) but there was no gas chamber or other execution method at Nordhausen.
