Need site to checkout in multiples of 6

Hi, I have had my woocommerce site up and running now for 6 months and originally was selling my individual products in groups of 6 using min/max maximiser plugin. However, I now need to change my site so that customers can mix their own products into cases and only need to purchase 1 of each variety, but, I now need the shopping cart to only allow multiples/denominations of 6 at checkout (we sell wine by case size of 6). I’m not a code writer, but I can take guidance if straightforward to do. Thanks!