materio-base-d7/sites/all/modules/contrib/ecommerce/uc_restrict_qty

Ubercart Restrict Qty lets you add a very basic Restrict Qty. to a product in your Ubercart store to limit the quantity of that product in the shopping cart to 1. This is useful for single purchase products or for instances where it just doesn't make sense for a customer to purchase more than one and you want to prevent it from happening.

Due to the way products are added to the cart, customers can add multiple variations of the same product to their carts (i.e. different attribute/option combinations) but never more than one of any given variation.

Instructions:

Upload the module to your ubercart/contrib directory and enable it on your Drupal site.
Browse to the product you want to have a quantity restriction as an administrator.
Click on the product's Edit tab and then its Features secondary task tab.
Choose Restrict Qty. from the select box and click Add.
Test it out by adding the product to your cart!
If you already had a copy of that product in your shopping cart, you should empty your cart and add it again. Due to the way the module works, it won't restrict quantities on products already in customers' shopping carts.