It depends. I've personally found a single instance of any Rails app
can deal with a pretty reasonable amount of traffic, but having a
little concurrency will help in most cases. Unless your blog is going
to be taking particularly high traffic, then at most you'll want a few
backends running. I'd only set min_instances to 1 though. You can then
set the timeout to, say, 10 minutes, which is a sweet point unless
you're running in a particularly memory-thirsty environment (in which
case I'd go down to perhaps a minute.)
Cheers,
Pete