I once used OS 8 on an LC580 with 12MB of RAM. It booted, and sort of ran, but it was terrible, I wouldn't recommend it at all.

FWIW, OS 8 doesn't actually do a check at startup to see how much RAM you have - I've actually booted OS 8 on a machine with 4MB of RAM before...with extensions off, it boots....and then runs out of memory when it tries to load the Finder.