At product page of ACS has been written the sentence bellow. what does it mean 50 AAA clients that supports only 50 devices to be configured to use AAA?
Cisco ACS Express is available as a 1-rack-unit (RU), security-hardened appliance with a preinstalled Cisco Secure ACS Express license. Cisco ACS Express supports a maximum of 50 AAA clients and 350 unique user logons in a 24-hour period
If the license is for 50 clients then you can only query ACS express with 50 devices. If you have more than 50 devices that need ACS then you need something more than ACS Express (or you need 2 ACS Expresses).