There are different sources about the number of prisoners. They count up to 20000, but a total number of prisoners between 12000 and 15000 are more likely. There were different workingplaces; they had to work by building railroads, excavation work in the Basement of the Mühlburg and the "Four Colour Castle" (A colour producing plant, located in an old castle near Ohrdruf). Also, as known, in the Jonastal.
But that can not be all. If one looks at an example, it becomes clearer:

Comparison

	Magnesit	Schwalbe	S III
Prisoners	800	500	12000
Tunnellength	4000 meter	3000 meter	2900 meter
Build in	Molasse	limestone	limestone

As one can see, the Building place with the most prisoners had the smallest Tunnellength. And this, although S III was known as one of the hardest Camps.
This means, that at least one system is unknown. But two or three are likely. One of them must be the, by many prisoners described, system with "large Halls" and "wide caves".
An attempt, to locate it, is difficult for the reason of the poor archival situation. Also, prisoners never talked about exact locations in the testimonies.
But they often described their work as "work in underground quarrys". Near Gossel are Limestonequarrys, and it was often usual, to dig in tunnels, to use the inner Limestone, which had a better quality. So the term "work in underground quarrys" could have a real sense. Another probably place is a Sandstone Quarry near Crawinkel, where the same could have happened as near Gossel; dig tunnels in, and excavate. To this place a testimonie fits; H. Zuckermann, an inmate of Crawinkel described: "The camp was near the work site. We had to dig many tunnels and caves in the mountains, to protect projected factorys against Air attacks. There should be produced V-Weapons".
If the work site was near the camp, the Sandstonequarry fits; it is about 2 km from the camp, and there are several positions, which could mark blasted tunnelentrys. Without technical equipment as Protonenmagnetometers or Gravitometers there is no chance, to proof that.

A last probably place is near Camp Ohrdruf, on the Military Training area. Some engineers told, the Fuehrerheadquarter was located IN the military training area.(I have marked the possible places with blue squares).

As one can see, there are lots of unsolved questions.