Installs may either have large numbers of mapped local realm (source) users (NEXUS-19209) or have their NXRM connected to one or more external realms with many users (LDAP). In some cases an NXRM instance may have more than 100 total user records, even in the default realm.
Currently the /security/users REST API is capped at returning 100 users. ( see NEXUS-21389 for more specific limitations )
Implement ability to page through users returned by /security/users/.
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 of a repository manager.