From: Arad, Yitzhak; Belzec, Sobibor, Treblinka, pp. 381-382 (Appendix A):

The Deportation of the Jews from the General Government, Bialystok General District, and Ostland

The exact number of Jews who were deported to the Operation Reinhard death camps is difficult to determine because of the prevailing conditions at the time and the method employed by the Nazi extermination machine in expelling the victims to BeIzec, Sobibor, and Treblinka. The number of Jews who lived in the towns and townships of Poland before the war is known from the population census carried out there in 1931. Some demographic changes took place during the years 1931-1939, but these did not basically alter the number of Jews living there on the eve of the German occupation.
Substantial demographic changes did occur during the war, during the years 1939-1942, until the onset of the deportations to the death camps. In these years, tens of thousands of Jews escaped from one place to seek refuge in another. Hundreds of thousands of Jews were expelled and resettled, sent to labor camps, or concentrated in larger ghettos. Thousands of Jews were murdered in shooting Aktionen in the vicinity of their homes-before, during, and after the deportations to the death camps. Thus, on the eve of the expulsions, there were many small localities in which Jews no longer lived and other localities in which the number of Jews was much higher than before the war.
The deportation method, as carried out by the German authorities in the General Government, was en masse, without lists of names or even exact numbers. Usually ghettos were totally liquidated, and only the killing capacity of the camps and the volume of the trains dictated the number of people who were deported. In places where some Jews were temporarily left behind, the Germans counted the few who remained, while all the others were pushed into the trains.
Documents of the German railway authorities, which were found after the war, provided some data on the number of trains and freight cars. If we take into account that each fully packed freight car carried 100-150 people, we can arrive at an approximate indication of the number of Jews in each transport.
Another source of information was the census of the ghetto inhabitants carried out by the Judenrats in some of these places. A census of this type was usually undertaken by order of the German authorities for purposes of forced-labor requests or in preparation for the deportations. Sometimes the Judenrats also took a census for their own purposes, for example, for food rationing or housing problems.
Documents containing these data and sometimes even the number of Jews who were deported, as collected by the Judenrat, were found after the war. Sometimes they were mentioned in diaries written by ghetto inmates and left behind.
Numerous memoirs written by survivors, as well as the memorial books (Yizkor books), contain important data about the deportations, including, dates and the number of deported. Testimonies by survivors, statements by local people who witnessed the deportations, and evidence given by members of the German administration at the war-crimes trials serve as significant sources of information.
Together, all these documents and sources enable us to arrive at an estimation that comes very close to the actual figures and dates of the deportations to the Operation Reinhard death camps.

An extremely valuable research study undertaken to establish the timetable and number of deported Jews from the General Government and to which death camp they were sent was carried out by Tatiana Berenstein and published in Poland in the Biuletyn Zydowskiego Instytutu Historycznego (Bulletin of the Jewish Historical Institute), Warsaw, No. 3/1952, No. 21/1957, No. 3/1959, No. 59/1966, No. 61/1967. Another source is the "Luach Hashoa (Holocaust Calendar) of Polish Jewry" prepared by Rabbi Israel Schepansky and published by "Or Harnizrach," New York, 1974. A most important and more up-to-date source is the Pinkas Hakehillot (Encyclopedia of Jewish Communities), Poland, Vol. 11, Eastern Galicia, and Vol. III, Western Galicia, published by Yad Vashem, Jerusalem, in 1980 and 1984. The following tables of the deportations are based on all the aforementioned primary sources and research studies.