If you install 20 subcommunities in one OpenACS instance, a clumsy or malicious person working on one subcommunity could destroy the other 19 from PSQL or SQL*Plus.
So for security reasons alone, installing 20 separate instances under 20 different database users would make the most sense if you're talking about hosting 20 people who don't know and trust each other.
You're asking an implicit performance question, too. I don't know that answer to that but I'm interested in it.
