Setting a Maximum Purchase Limit for a product:
Log in as Admin.
Under Shopping Cart >> Product Maint, you will see the 5th section "Product Details":
Click 'add new' then choose 'Maximum Purchase Allowance' from the droplist (as shown above).
You may enter a limit under Value which specifies the maximum lifetime limit for purchases of this product. Users will not be able to purchase more than this amount. This is a cumulative limit for the lifetime of the user's account.
Click the 'save product details' button.
The "Processed" window will display to show this has been done.
How this operates:
When making a purchase, the shopping cart checks to see how many times the person has previously purchased this product.
The shopping cart also checks pending orders. So the total number checked against is (purchases + pending). If this number equals the limit, then further purchases are prevented for that user.