There are two ways of changing the quantity of product licenses you are adding to your Cart.
The first opportunity you have to do so is in the "Add to Cart..." screen, which is displayed once you have selected to add a product to your Cart.
To do so, highlight the number in the "Quantity Required" box and enter the amount you require. Then click on the "Add to Cart" button to update the amount of licenses you wish to purchase and continue to the Cart.
The other stage during which you can change the license quantity you are purchasing is in the Cart itself.
Here you need to highlight the "Qty" box and change this to the amount of licenses you require. Once you have done so, use the "Recalculate" button to update your Cart.