Видимо документацию нынче читать не модно (
https://msdn.microsoft.com/en-us/library/system.th...
GetAvailableThreads(Int32, Int32) - Retrieves the difference between the maximum number of thread pool threads returned by the GetMaxThreads method, and the number currently active.
Дальше, надеюсь, догадаешься сам.