For redundancy purposes, multiple Enrollment Servers (ESs) can be added to each Site. All enabled and verified ESs will be used in an active/active fashion.
Upon user login, requests from workload VMs such as RD Session Hosts or VDIs are equally distributed among the available ESs. In case of failures on a particular ES, the next available ES is selected and the SAML SSO authentication process continues. Specifically required for manual deployment of multiple ESs, it is important to note that all ESs in the same site share the same registration key which is required to be deployed in the specified path as mentioned in the RAS Enrollment Server Configuration section (p. 266 of the Admin guide)