Our experience mirrors Ben's; we've got 10 instances on a
system with 512 Mb of RAM, and that looks to be the limit.
However, for sites which aren't too busy you can turn off things
like fastpath and adp caching, which will reduce the amount of
memory each nsd instance requires, and probably squeeze in a
few more. (thanks to Don for this suggestion, which I'm just now
starting to experiment with)
BTW, the load average on this system is 0.0, proving once again
that OpenACS, postgres and nsd are incredibly lightweight. I'm
pretty confident that if we maxed out the memory in the system (2
Gb, I think) and ran 40 small sites, we'd still see very little load.
The system has a single 733 Mb PIII, outdated by today's
standards.
