@Override
    public void execute(@Nonnull final SessionData data, Element command) {
        final Locale preferredLocale = SessionManager.getInstance().getLocaleForSession(data.getOwner());

        DataForm form = new DataForm(DataForm.Type.result);

        FormField field = form.addField();
        field.setType(FormField.Type.hidden);
        field.setVariable("FORM_TYPE");
        field.addValue("http://jabber.org/protocol/admin");

        field = form.addField();
        field.setType(FormField.Type.text_single);
        field.setLabel(LocaleUtils.getLocalizedString("commands.admin.getnumberregisteredusers.form.field.registeredusersnum.label", preferredLocale));
        field.setVariable("registeredusersnum");
        field.addValue(UserManager.getInstance().getUserCount());

        command.add(form.getElement());
    }
Command that allows to retrieve the number of registered users

@author Guus der Kinderen, guus@goodbytes.nl
@see <a href="https://xmpp.org/extensions/xep-0133.html#get-registered-users-num">XEP-0133 Service Administration: Get Number of Registered Users</a>
xmppserver/src/main/java/org/jivesoftware/openfire/commands/admin/GetNumberRegisteredUsers.java