I understand now. If you go to your Products_Details.php page and find this line:
$ATC_itemQuantity = "".((floatval($_POST["WA_Store_Cart_1_Quantity_Add"])<=floatval($row_NumLeftRS['NumLeft']) || $row_WADAProducts['ProductUnlimited'] != 0)?$_POST["WA_Store_Cart_1_Quantity_Add"]:$row_NumLeftRS['NumLeft']) ."";// column binding
You should modify it to look like this:
$ATC_itemQuantity = "".$_POST["WA_Store_Cart_1_Quantity_Add"] ."";// column binding