Quantity Control allows you to specify a maximum quantity for each product in the cart. This is a simple module - just install and browse to admin/store/settings/qty_ctrl to set the maximum quantity allowed in the cart for each product class.
This module is similar to UC Restrict Quantity. The key differences are:
1. The module takes into account both what is in the cart as well as the number being added
2. The maximum quantity for each product is set at the product class, so it isn't necessary to add the setting for each product created. This can save time when there are many products being added.
3. Restrict Qty has a maximum of 1 item for each product in the cart - this module allows you to set the maximum to be whichever number you want.
This modules is for Ubercart on D5 but could be modified relatively easily for D6.