One day I'll catalogue what I actually have, but it occurred to me me that not only have I forgotten many of the rpg system rules of the games I used to play and run (RoleMaster / SpaceMaster / MERPS, WFRP, the Pacesetter System, RuneQuest, Traveller, CoC, Gamma World, GURPS, Champions, Rifts and other systems that escape me at the moment) but I have a ton of systems I just never learned.
Then we look at the more recent acquisitions and with few exceptions they just aren't being played. Most of the OSR stuff can be considered a single system, just with different tweaks, so my preference for Swords & Wizardry means I can still grab adventures from Castles & Crusades right on through Dungeon Crawl Classics and over a dozen systems in between - but I generally only run S&W as my OSR system of choice and convert from other OSR systems on the fly.
Tunnels & Trolls is simple enough (once you get past the horrible organization of T&T 5e) that I can run it without referring much to the book. Far Away Land is the ONLY recently published rule system that I grokked right away and can run with minimal if any rules searching.
That's it. D&D derivatives, T&T and FAL. Three systems that I can grok these days. Dozens if not hundreds that I own and will never play - either for the first time or the first time in 20 years.
How many systems do you actually play compared to owning?