FortiAuthenticator - licensing of TACACS service for client defined as a subnet.

Hello team. Could you please help me to clarify the FAC licensing of TACACS service? The documentation for VM says that Radius/Tacacs clients are licensed as user licenses/3. I would like to define a single Tacacs client as a subnet /14. Will be consumed just a single license for all network devices which act as Tacacs clients in the /14 subnet (I am fine with just FAC BASE SKU) or do we need a single license for each network device (500 network devices so 1500 users licenses)? Thank you very much.