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add smarty variable
 

$rows = Db::getInstance()->executeS('
			SELECT id_product
			FROM `'._DB_PREFIX_.'cart_product`
			WHERE `id_cart` = '.(int)$this->context->cookie->id_cart);
// or distinct products        
//$rows = Db::getInstance()->executeS('
//			SELECT DISTINCT id_product
//			FROM `'._DB_PREFIX_.'cart_product`
//			WHERE `id_cart` = '.(int)$this->context->cookie->id_cart);

$rows = ($rows) ? $rows : array();
$this->context->smarty->assign('cart_rows'=> count($rows));

and use it:

<span class="ajax_cart_quantity mini-cart-count{if $cart_rows == 0} unvisible{/if}">{$cart_rows}</span>