I'm talking about the minimum. Should one set to 20 or should we allow the end user to configure it? This are important performance parameters when tuning web applications.

As so, the maximum, it's seems a bit wrong to allow it to grow without restrictions, the JVM/machine could not support the given number of threads and still the application should be capable of running without blowing up.

Apart from that, if one decide to establish configurable values, I would say we should allow also configuration on a per request basis too, but that could postponed.