By default it has 25 Main threads.
Is that per server or per CPU?
Per Server

When do you increase that number if you are running on 4 cores?
No definite answer here - it's a matter of trial and error. Too many threads may cause database deadlocks I suppose. On a 4 core server, I would push it up to say - 100 and see how it behaves.

What's the difference with the Main thread set to k2 .net vs .NET Frameork?
No real difference - all part of the same thing. We do use our own threadpool though.

When would you want to set the Log Thread from Low to Normal? (Default is Low)
Depends on the priority or frequency of reporting. If you need actions to be logged IMMEDIATELY for reporting purposes, change the Log Thread's priority to High. If you can wait a couple of seconds or in extreme cases a couple of minutes, leave the Log Thread's priority on Low.

HTH,
Ockert