In /wp-content/mu-plugins/domain_mapping.php, I changed this line:
$rows = $wpdb->get_results( “SELECT * FROM {$wpdb->dmtable} ORDER BY id DESC LIMIT 0,20” );
to:
$rows = $wpdb->get_results( “SELECT * FROM {$wpdb->dmtable} ORDER BY id DESC LIMIT 0,250” );
It now shows all my sites in the list..