What is the maximum number of authentication sources that can be added to a service in ClearPass?

ClearPass Policy Manager supports a maximum of 23 authentication sources per service. This limit however is not currently enforced by UI.

Configuring more than 23 Authentication Sources in a service will stop the RADIUS Service.

Solution is to remove some authentication sources so the number of Authentication servers count is equal or less than 23 and start the RADIUS Service manually using the below command via appadmin access:
 

Or from WebUI of ClearPass Server:

Administration » Server Manager » Server Configuration » Click on CPPM Server » Click on Service Control tab » Hit "Start" next to "Radius Server" to start the RADIUS Service

Note: Having too many authentication sources is not recommended as this will impact the RADIUS authentication processing time especially if the user is present in an authentication source down in the list (as of 6.5.5).