No real idea, I've never seen it discussed.
However, as long as you never run out of database handles, I doubt
there's that much performance impact of how many you have. There's
overhead (particularly on the Oracle side) for each handle, so
probably the fewer the better, without running out, but I don't know
how much overhead there is per handle.
Note that with the db api in OpenACS 4.x, the 2nd and 3rd pools
(traditionally called "subquery" and "log", although they no longer
have those meanings), will be used only for the unusual
queries which need multiple handles. Therefore, I suspect that the
vast majority of your handles should be in the first "main" pool,
not the second or third pools.
