The options that I see is adding default value for the Quantity and validator to it with a buttons that add or remove only by 6 items to the shopping cart.
You can override AddToCartWidget control. Inside its InitializeControls method you can set the default value to 6
this.QuantityControl.Text = "6";
Then you need to implement the validator and the buttons that will change the text of the QuantityControl with 6 items. You can avoid implementing the validator by disabling the text editing of QuantityControl, so it will be updated only with buttons that will add/remove 6 items per click.
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