That's really a tough question. To give you some perspective: my 486DX33 was exceptionally well equipped with 32 MB RAM but to be honest: no game I had
could possibly utilize so much memory, nor could Windows 3.11. It would have probably been the same experience with "only" 16 MB RAM.
The Pentium 90 that was handed down to me later on had only 16 MB RAM with Windows 95 C and I was pretty happy with it, playing Diablo and Civ 2 mostly.
And that's exactly why I'm saying this is a hard question to answer: it's all about perception. Back then there was normally no mindset like "60, 120, 144 FPS or else"
because it was much harder to compare game experiences from different machines as there was no YouTube.
Even my Duron 75o later on only had 64 MB RAM running on Windows 98 SE (granted, only for a short while, then I got additional 64 MB RAM) and to me that seemed normal
and I played Counter Strike and the like with friends on lan partys. It didn't even occur to me that my gaming experience might be worse than that of my friends.
What I go by nowadays is in every way not realistic for what most of us had back then but my currently active retro machines have the following amounts of RAM, along with the Operating Sytems I'm using and some games I'm playing:
1) 486 DX2 66 with 24 MB RAM running DOS 6.22 and Windows 3.11. Games: Warcraft 1 and 2, Realms of Arkania 1-3, Alone in the Dark 1-3, Descent I and Civilization 1.
2) AMD K6-2+ with 256 MB RAM running Windows 98 SE. Games: Command and Conquer Red Alert 1 and 2, Tiberian Dawn, Tiberian Sun, Half-Life, Descent II and some adventures.
So, a 386 would probably be more than fine with 16 MB RAM. Lower machines are having also lower compute capabilities and as such are most likely not as much limited by less RAM as they can't
properly handle more demanding applications anyways. In some cases you even explicitly need a 386 or later for running certain applications.
And all in all Baoran's answer might not be to your (or somebody else's liking) but it is mostly true: it comes down what you actually want to do with your machine. While more RAM doesn't hurt it might simply go unused.