Shopping Cart Only Storing one Product and not Increasing The Cart ID

Hello everyone I am working on an eCommerce site and need some help. Currently, I have some products that are on display for sale. By each item there is a box where a person can enter how much of the product they want and then press the button to add it to the cart. I had one item get added into the cart just fine. However, when I try add another item into the cart it either doesn't get added or it adds the first item again and not the new product I clicked on.

Here is code to show the products:

  <?php
            $consoles = getConsoles($db);
            if(!empty($consoles)){
                foreach($consoles as $key=>$value){
    ?>

    <form method="post" action="index.php">
    <p><?php echo $consoles[$key]["name"]; ?></p>
    <p><?php echo $consoles[$key]["description"]; ?></p>
    <img src="<?php echo $consoles[$key]["image"]; ?>">
    <p><?php echo "$" . $consoles[$key]["price"]; ?></p>
    <p><?php echo $consoles[$key]["quanity"]; ?></p>
    <p><?php echo $consoles[$key]["limted_Edition"]; ?></p>
    <p><?php echo $consoles[$key]["year_Published"]; ?></p>
    <input type="text" name="q" value="1" size="2" />
    <input type="hidden" name="serial" value=<?php echo $consoles[$key]["serialNumber"]; ?> >
    <input type="submit" name="add" value="Add to Cart" />
    <?php 
                }

            }


    ?>      
    </form> 

Here is my code for added to a cart:

         include 'phpfunctions/functions.php';
         include 'open_db.php';

         $number = 1;
         if (isset($_POST['add'])){
          $number++;
          $cartID = $number;
          $username = "Twili";
          $itemid = NULL;
          $serialNumber = $_POST['serial'];
          $quanity = $_POST['q'];
           addToCart($db, $cartID, $username, $itemid, $serialNumber, $quanity 
          );
        }

Thank you for any help!