Who were the "Hiwis" during World War II? How many were they and which countries did they come from?

This is a difficult and controversial subject.

In the context of the war, "Hiwi" (from German "Hilfswilliger") meant a "volunteer" in a German-occupied country. Such volunteers often handled wuite innocuous tasks, but some were involved in rounding up undesireables (read: Jews) for deportation:


https://en.wikipedia.org/wiki/Hiwi_%28volunteer%29

The forerunner of the volunteer formations was a voluntary auxiliary service, of a para- military character, which was started in the autumn of 1941 by the German Commands on the front. On their own initiative, they organized auxiliary units of various services, made up of Soviet deserters, prisoners, and volunteers from among the local population. These so-called "Hilfswillige," or "Hiwi," were employed as sentries, drivers, store- keepers, workers in depots, etc. The experiment surpassed all expectations. In the spring of 1942 there were already at least 200,000 of them in the rear of the German armies, and by the end of the same year their number was allegedly near 1,000,000.

http://www.feldgrau.com/rvol.html