To add multiples of an item, simply head to your shopping cart once the item has been added, and you will then be able to change the quantity. Make sure you hit "update my changes" at the bottom of the page to ensure your updates are saved.
If you are finding that a quantity will not change, or reverts back to another number (i.e, changing quantity to 10, but it reverts back to 8), this likely means the item is low in stock. Feel free to contact us for availability!