if( $config['allow_registration'] != "yes" ) {

msgbox( $lang['all_info'], $lang['reg_err_9'] );
$stopregistration = TRUE;

} elseif( $config['max_users'] > 0 and $row['count'] >= $config['max_users'] ) {

msgbox( $lang['all_info'], $lang['reg_err_10'] );
$stopregistration = TRUE;

}

заменить на:
if( $config['allow_registration'] != "yes" ) {

msgbox( $lang['all_info'], $lang['reg_err_9'] );
$stopregistration = TRUE;

} elseif( $config['max_users'] > 0) {
$row = $db->super_query( "SELECT COUNT(*) as count FROM " . USERPREFIX . "_users" );
if ($row['count'] >= $config['max_users'] ) {
msgbox( $lang['all_info'], $lang['reg_err_10'] );
$stopregistration = TRUE;
}

}