Navigate to %LOCALAPPDATA%\Gas Powered Games\Supreme Commander Forged Alliance\ and delete the cache folder. This forces the game to index the 600 new maps.
Do not trust random torrents from 2012. Go to the official Supreme Commander subreddit or the FAF forum "Maps" section. Look for the post titled "600 Map Pack Updated (FAF v6.0 Compatible)." The file size is approximately 4.2 GB compressed.
For over a decade, Supreme Commander: Forged Alliance has remained the gold standard for large-scale real-time strategy. While the base game shipped with a respectable collection of battlegrounds, the true longevity of this legendary title comes from its community-driven content—specifically, map packs.