src/Eccube/Controller/ProductController.php
・index()のforeachの最後に以下を追加。
            $stmt = $app['orm.em']->getConnection()->prepare('SELECT COUNT(*) as count FROM plg_product_review WHERE product_id = '.$Product->getId().' AND status = 1 AND del_flg <> 1');
            $stmt->execute();
            $result = $stmt->fetchAll();
            $arrReviewCount[$Product->getId()] = $result[0]['count'];

・最後のrender()のarrayのパラメータに以下を追加
            'arrReviewCount' => $arrReviewCount,

template/default/Product/list.twig
・{{ arrReviewCount[Product.id] }}で表示