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 from cli.
Or from WebUI of CPPM Server:
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.