During the Cold War, the Military Topographic Directorate (VTU) of the General Staff of the Soviet Army conducted a secret topographic mapping program at a high level of detail and coverage for almost the entire globe. The true extent of the Soviet cartographic enterprise is yet to emerge, but it is clear that this was the most comprehensive global topographic mapping project ever undertaken. The number of different maps produced is impossible to quantify, but it is certainly well in excess of two million.
The map series can be classified as:
Topographic maps (Topos):
* Military series (SK-42)
* Civil series (SK-63)
City plans:
* Military series
* Civil series
Special maps: Other series, such as 1:300,000 topographic maps, large-scale small-town plans, aeronavigation maps and rectangular topographic maps.
КАРТА МИРА (Karta Mira), the non-secret 1:2,500,000 World map produced jointly by seven Warsaw Pact nations and available for sale in the West
The best available source of information is The Red Atlas, by John Davies and Dr Alexander J. Kent, published by the University of Chicago Press in 2017.
Those interested in the maps' historical and geopolitical context are urged to read the book, which also provides extensive descriptions of how and why the maps were made, with copious illustrations. 
This website supports the book by providing useful details, examples and listings.
Print codes
Map numbering scheme - IMW
US Army guide (1958) to Soviet Topographic Map Symbols
Other useful links and references