Here case answer:
The threshold values are warning 2000/min critical 4000/min Number of requests coming in to NAC from per switch with in a minute, so this is calculated on per switch basis.
The appliance properties to override them are:
warning RADIUS_REQUEST_WARNING_THRESHOLD
critical RADIUS_REQUEST_ALARM_THRESHOLD
Right click on the Engine groups from the Access control tab >> Engine properties and add the needed values accordingly.
Restart of the NAC service is required for this value to take effect.
Regards