Richard,

I've got a production instance running over 30 databases, and development
ones running over 50.   Most of theses are small databases which are fairly
lightly used; if you had something which was very large or very heavily
utilised then I'd give it a dedicated instance.

I'm not sure there is an "official" IBM recommendation, but I think in
general they'd consider that 30 databases was too many for one instance !!!

Some things I've found -

Some tools expects that you've only got one database in an instance,
especially some of the tuning ones.  In fact you'd almost think some of
them only expected you to run one database per box : you'd think the tools
were written by a hardware vendor ... oh wait, did I miss something ;-)

Multiple databases per instance is more memory efficient than lots of
instances with one database each.   The new STMM (Self Tuning Memory
Management) seems to work fine with multiple databases in an instance.  I'm
not sure that WLM (Work Load Management) is as happy about this.

There have been reports that the new thread-based model is less tolerant of
high numbers of databases per instance than the old process-based one was,
especially for 32-bit instances.   That should not affect most people these
days however.

HTH

Phil Nelson
ScotDB Limited