I've never had more than two dozen or so hard copy books at any one time. The most books I've had for any system was 3.5.
About four years ago I sold pretty much everything that I knew I wasn't ever going to GM ever again. So all my 3.5 and Pathfinder stuff went, other than campaign settings.
Probably the only sale I really regret was getting rid of my original 1990 Rules Cyclopedia. So many great memories of playing that system, but once again, I knew I'd never run it now.
At this point my books can fit on to a single, 3 foot wide shelf. It's a hodge podge of Savage Worlds, One Ring, a few random others (Gumshoe, Fate, Genesys, Fantasy Craft, d6 Star Wars, Dungeon World), and maybe 3 or 4 modules / Paizo AP booklets.
And yet even with that small of a collection, I still feel like I have enough RPG material to last me the rest of my life.