Decide what you want to count
A total item-count rule checks the total quantity of items in a cart. A selected-product quantity rule checks quantities for specific products. Choose the rule that matches your policy rather than treating the two as interchangeable.
Variants contribute to a product total
Suppose a product has a per-cart maximum of four. A shopper adds two small and three medium variants. The selected product total is five, which exceeds that maximum even though neither variant individually exceeds four.
A per-cart limit has a defined boundary
A maximum quantity can support a limited-item or sample policy for one order. It does not by itself prevent the same person from creating several orders. Communicate your policy accordingly.
Test presence, absence, and mixed carts
Test one variant, multiple variants, duplicate lines, and a cart containing unrelated products. Also test a cart without the selected product. The selected-product minimum applies to a present product; it should not require an unrelated cart to contain that product.
Make the message specific
Tell shoppers which product or quantity needs to change. For example: “Choose up to four of this product per order.” Confirm the message still makes sense when the quantity is spread across variants.