Idle indicates the number of threads that are currently idle. Peak indicates the peak number in the pool. Limit indicates the maximum number of native threads allowed in the thread pool, and is determined by the setting of NativePoolMaxThreads.
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