One can use the /service/rest/beta/security/users?source=LDAP resource to list up to 100 users per LDAP server at most. ( see NEXUS-21389 for specific limitations ). In some cases an NXRM instance may have more than 100 total user records, even in the default realm.
Support paging through the the list of users if more than 100 for the specified source, until all of them are read.
If the goal is to list all users NXRM has access to in an external realm such as LDAP, then the recommended way to do that is query your LDAP server directly using LDAP query tools.
Using NXRM as an LDAP user directory browse tool is not a core feature.