I believe the simplest solution is to have 4 product(sprinkle, chocolat, maple, minty) add to your cart.
Have a custom add to cart button that is enabled only when the total quantity of donuts is equals to 6 or 12.
Once the user client add to cart, add each donut with its quantity. Example
5 Sprinkles + 1 Chocolat.
Key is to not let user pick a number of donut that not a multiple of 6.
You’ll need to do the same thing on the checkout page so that user can’t remove or add 1 donut to their cart on the checkout page.