Returns how many threads have the given VortexThreadPool.
This function is for internal vortex library purpose. This function returns how many thread must be created inside the vortex thread pool which actually is the one which dispatch all frame received callback and other async notifications.
This function pay attention to the environment var "VORTEX_THREADS" so user can control how many thread are created. In case this var is not defined the function will return 5. In case the var is defined but using a wrong value, that is, an non positive integer, the function will abort the vortex execution and consequently the application over it.