none, there is only a museum and memorial there now.
525,000 people died in Belzec Concentration Camp.
1000,000
Chelmno and Belzec came into operation as an extermination camp a few months before Auschwitz II.
Sonderkommandos were at the death camps: Auschwitz, Birkenau, Sobibor, Treblinka, Belzec, and Chelmno. Though every camp needed burial and corpse disposal squads (as there were many deaths from starvation/disease, exhaustion/beating and euthenasia).
There were six extermination (or 'death') camps in the Holocaust which were located at: Auschwitz-Birkenau, Belzec, Chelmno, Majdanek, Sobibor, Treblinka. That is the 'accepted list', but the role of Majdanek is not clear and there was also an extermination camp at Maly Trostinets near Minsk.
By 1941, the Nazis began building Chelmno, the first extermination camp (also called death camp), in order to "exterminate" both Jews and Gypsies. In 1942, three more death camps were built (Treblinka, Sobibor, and Belzec) and used solely for mass murder. Around this time, killing centers were also added at the concentration camps of Auschwitz and Majdanek. So i would say 6 or more.
The link below even gives the names of the German guards at Belzec.
500,000
23
about 900,000
Maybe a few hundred.
about 50,000-100,000