Knowledgebase
How to set a minimum number of units from a product to be added to the shopping cart in Magento
If you want to, in Magento you can set a minimum number of units to be ordered. For example, you may want customers to be required to purchase at least two units of a particular product. You can set a minimum quantity globally, so that it applies to all products in your shop, or you can set a minimum quantity per product.
To set a minimum quantity that applies to all products, log in to the admin panel and go to System menu>Configuration>Inventory button in the Catalog section on the left>Product Stock Options panel on the right. Expand the panel and click on the Add Minimum Qty button that's there. A couple of options will appear. There's a drop-down menu from which you can select the customer group to which you want the minimum quantity to apply, and in the field next to the drop-down menu type the number for the minimum quantity. You can add different quantities for the different customer groups, or you can set the minimum quantity to apply to all customer groups. After that click on the Save Config button. Don't forget that this will apply to all products.
If you don't want to set a minimum quantity for all products, you can do that per product. To do this you need to edit the settings of the particular product. In the admin panel go to Catalog menu>Manage Products, select the product that you want to edit and on the page with the product settings click on the Inventory button on the left. Then on the right find the setting Minimum Qty Allowed in Shopping Cart, unmark its Use Config Settings checkbox and type the number for the minimum quantity in the text field. Don't forget to click on the Save or on the Save and Continue Edit button.
When you set a minimum quantity for a product, the quantity option for that product on the frontend will be set to that number automatically. So, for example, if it's set to 2 and the customer doesn't modify the quantity and clicks on the Add to Cart button directly, two units of that product will be added to the shopping cart. The customer can put more units of the same product in the cart, but they cannot add fewer units than the specified minimum quantity.
If you want to, you can also set a minimum order amount. For more information read the article on setting a minimum order amount in Magento.