private static List<String> nnUniqueIds(String nameServiceId, ConfigHelper config, Cluster cluster) {
    return separateByComma(config, cluster, "dfs.ha.namenodes." + nameServiceId);
  }
NameNodes in this nameservice.
The minimum number of NameNodes for HA is two, but more can be configured.
ambari-server/src/main/java/org/apache/ambari/server/stack/NameService.java