All I did was ask if the numbers were accurate. And if so, then how was that number was verified. And if not then how many were actually killed and how was that number verified.
You might have come across the fact that the six million figure was mentioned in the Nuremberg judgement, based on the testimony of an SS officer, Wilhelm Hoettl, who said he heard this from Eichmann in 1944. In fact Hoettl said that Eichmann told him 4 million died in the extermination camps and 2 million died elsewhere by shootings or in ghettos.
This is the origin of the 'canonical' 6M figure, and it doesn't come from "the Jews" at all. But the order of magnitude does not rest on the hearsay of one SS officer alone.
Further digging would have revealed that another SS officer, Dieter Wisliceny, also testified at Nuremberg that the same informant, Eichmann, had told him five million Jews had died. He spent a long time with interrogators going through country by country the numbers he had heard, being very careful to distinguish between what he knew with more certainty and what he knew less well, who his informants (multiple) were, and so on.
So already two sources give an estimate range between 5 and 6 million, and they happen to be right - the number is in between 5 and 6 million.
They are right because Hoettl and Wisliceny were not the only two sources giving figures in the years immediately after the war. The 'verification' comes from contrasting these sources, since we are dealing with a transnational event affecting Denmark, Norway, Belgium, Netherlands, Luxembourg, France, Italy, Greece, Bulgaria, Yugoslavia, Albania, Austria, Germany, Czechoslovakia, Hungary, Romania, Poland, and the Soviet Union.
Firstly, one can see that the states that liberated the killing sites reported estimates of the number of victims, without necessarily distinguishing between Jewish and non-Jewish victims. That is less of a problem when looking at estimates for Treblinka since virtually only Jews died there, more of a problem for Auschwitz at this time, since many Poles died there. The initial estimates were provided by the Soviet and Polish states, not Jews.
The 'official' estimates were overestimates:
Auschwitz 4 million
Majdanek 1.5 million
Together these camps add up to 5.5 million, but the victims were not all Jews, and indeed the exaggerations can be attributed in part to a desire by the Soviet and Polish states not to put too much emphasis on Jewish victimhood and to drown it out in universalised suffering.
Historians, beginning with two Jewish historians, Gerald Reitlinger and Raul Hilberg, in 1953 and 1961, reduced these figures by using Nazi documents to circa 1 million Jewish victims at Auschwitz, and 58,000 out of 78,000 Majdanek victims being Jews. The main work in recent decades was done by Polish historians, Franciszek Piper for Auschwitz and Tomasz Kranz for Majanek, the latter making significant use of a document found by the British historian Steve Tyas.
The other main death camps had almost entirely Jewish victims, and they were estimated in 1945 as follows by the Polish state, based on limited documents and estimating numbers of transports per day multiplied by how long the camp functioned:
Belzec 600,000
Sobibor 250,000
Treblinka 781,000
Chelmno 345,000
Total, 1,726,000
These camps claimed mostly Polish Jewish victims. So Polish Jewish and Polish historians checked them by looking at the known deportations. Western historians and West German courts also started using the Korherr report, a very important Nazi document, so Chelmno was reduced to 152,000 based on this source. The same British historian Steve Tyas mentioned above found an intercepted Nazi radio message with cryptic numbers for B, S, T and L (for Lublin, ie Majdanek) that then reduced the other camps to 1.274 million for 1942, and correspondingly reduced the total.
The initial method wasn't so ludicrous - Belzec went from 600,000 to 434,000, which is 85%, and instead of it being thought - by those not studying the regions properly and just going on the attention grabbing big camps - that nearly all victims from the affected regions had died in Belzec, historians (often German) noticed that many had been shot on the spot alongside deportations.
In 1945-46, Polish Jewish historians who had survived the war compared and contrasted the number of Jews estimated prewar with the number of Jews counted as survivors, and factored in survivors still living elsewhere, e.g. exiles in the USSR, to estimate 3 million Jews died in Poland. Other calculations relied on overestimates of the number of exiles in the USSR and came up with 2.7 million.
This 'subtraction' method was quite standard, and made sense in the immediate aftermath. It was supplemented by war crimes investigations by the Polish and Soviet states, and by historical research, increasingly by an international community of historians including non-Jews as well as Jews.
At Nuremberg, the prosecution mentioned one of the earliest country-by-country estimates calculated by the Institute of Jewish Affairs using the 'subtraction' method. This estimated 5.7 million missing, of whom it was thought at least 4.5 million had been killed. The estimate worked with very limited information from the press at a very early stage (July 1945).
Country by country, the estimates were corrected based on state-led war crimes investigations, missing persons enquiries, repatriation ministry statistics, historical research using Nazi documents, and have gone on being corrected to this day. Germany, for example, created a memorial book in the 1980s using the full statistical and archival apparatus of the German state crosschecking all names, and then after reunification updated it with newly available information.
There have been many overall calculations since the first one in July 1945, all going country by country, based on more and more information, combining addition and subtraction methods to check them against each other. For example, Gerald Reitlinger came up with one of the lowest estimates in 1953, calculating 4.2 to 4.5 million victims, but he was excessively conservative, especially regarding Poland and the Soviet Union. Raul Hilberg calculated 5.1 million in 1961. These were both before the end of the Cold War, which made examining the USSR and looking at detailed sources for the East Bloc much easier.
One problem in many calculations is defining the territories since there have been so many border changes. A 1991 study by German historians offered very good country level estimates, but the editor, Wolfgang Benz, then ignored border changes and overlaps, and simply added the numbers provided by specialist contributors up to 6.2 million. Without the overlaps the number would have been more like 5.7 million.
So there is now general acceptance, after multiple calculations, that the number is between 5 and 6 million.
By comparison to calculations regarding other incidences of mass death, this is pretty good - nobody knows the absolutely exact numbers for the Great Leap Forward, for the Soviet Union, for the Holodomor, for the Armenian Genocide. The variations for the USSR and China dwarf the magnitude of the Holocaust. One can produce wildly varying numbers for the Rape of Nanjing depending on how you define that incident.
Your mother's offhand remark, by the way, is true only for a very narrow slice of the Holocaust. Jewish councils had to provide statistical data on mortality in the bigger ghettos like Warsaw, Lodz and Theresienstadt. The records survived the war. They add up to about 150,000 deaths from starvation and disease in these three ghettos. But far from wanting to exaggerate the numbers, the Jewish councils recording the data would have had an incentive to downplay them in order to pretend some Jews were alive and claim rations. Except that wouldn't have worked so well, as the Nazis were undersupplying the ghettos with food, causing the starvation in the first place, and the exact division of rations was delegated to the Jewish councils. So the records are evidently accurate, based on checking them fairly thoroughly (these are some of the most-studied places in the Holocaust). The councils also recorded the numbers deported from these ghettos to the death camps - but the numbers were cross-checked by the Nazis, since the Nazis ran the railways, not the Jewish councils.
