CustomPlugin.php

    public function modifyEntrySources(&$sources, $context)
    {
        if ($context === 'index') {
            foreach ($sources as &$source) {
                if (!isset($source['label'])) {
                    continue;
                }
                $total = craft()->elements->getTotalElements($source['criteria']);
                $source['label'] .= ' ('. $total .')';
            }
        }
    }