The video is the culmination of half a year of work by organiser L4Bomb4, who collected the replays recorded in Trackmania United Forever and Trackmania Nations Forever, then converted and rendered 12,000 of them in the newer Trackmania 2: Stadium. “Unfortunately, the skins of the cars got lost in the process, but I could render with 12k cars at a pretty high quality without the game crashing,” writes L4Bomb4 on the video’s YouTube page. Another person, Danixks, edited the video and used a compositing technique to include 8000 further replays that would crash the game if rendered at the same time. The result is 20,000 cars on a single track.
Source: https://www.rockpapershotgun.com/2016/01/05/trackmania-20000-cars-project/