Let's say, you want to create a 10% discount promotion on items that are in shortage, for instance the ammo. You also want to sell below 10 boxes at one time. Basically, if your customer adds 12 items to cart, he/she will be able to purchase only 10 boxes with the discount, and 2 - with regular price. Here are the steps to set this promotion up:
1) Go to Products -> Promotions -> Promotions or press shift twice and search for Promotions for a shortcut.
2) Press Add in the Promotion Maintenance page to create a new promotion.
3) In the Rewards tab of the Promotion, please choose the item name from the dropdown list, indicate the maximum quantity, discount percentage, and check the box Req? Make sure, that the Order Discount Percent field is set as 0.00.
4) The customer will then see this on the website: 10 items with the discounted price, and 1 - for regular price.
If you just want to limit the total amount of items that your customer is able to put in their cart, please go to Products > Products > Products, search for the item you want to set that limitation. Under the details tab using cart maximum you can limit the amount they can put in the cart and order at one time.