NativePoolMaxThreads determine the maximum number of threads in the native (kernel) thread pool.
A higher value allows more requests to execute concurrently, but has more overhead due to context switching, so bigger is not always better. Typically, you will not need to increase this number, but if you are not saturating your CPU and you are seeing requests queue up, then you should increase this number.
You can modify the NativePoolMaxThreads by:
Editing the NativePoolMaxThreads parameter in magnus.conf
Entering or changing the value in the Maximum Threads field of the Native Thread Pool page in the Server Manager