To limit the amount of a card that is sold, you can set a max quantity for a product.

To do this, find the product in question and edit it.

From the Product Info tab, enter in a max quantity value.

Therefore, if you have 10 quantity of an item, and you set your max quantity to 3, your customer will only be able to purchase and add to their cart 3 of this product.

Note, this limits a customer per check out transaction.  This does not stop a customer from making another consecutive order to purchase more of these products.

To learn more about batch updating the max quantity for your products, please see this link here.