Simple is good, just return you a amount number.

<?php
	global $user;
	$order = commerce_cart_order_load($user->uid);

	if (!$order || empty($order->commerce_line_items)) {
		$quantity = 0;
	}
	else {
		$wrapper = entity_metadata_wrapper('commerce_order', $order);
		$order_total = $wrapper->commerce_order_total->value();
		$quantity = 0;
		foreach ($wrapper->commerce_line_items as $delta => $line_item_wrapper) {
			$quantity += $line_item_wrapper->quantity->value();
		}
	}
?>

and

<?php
	echo $quantity;
?>