Crisp reacted to paul2009 in Checkout in Multiples of 12
@Crisp to clarify, you could add code that prevents a customer reaching the checkout until they have 12 items in their cart. The problem is with the next step - the checkout page. On this page there are buttons to allow the customer to remove items from their cart. You cannot add code to the checkout, so you cannot override this behaviour.
Crisp reacted to ThompsonWebDesign in Checkout in Multiples of 12
I would look at alternative e-commerce solutions. Whilst it is possible to code a solution you cannot add any code to the checkout page on Squarespace. This means that customers would still be able to change the quantities in their cart on the checkout page.
In my opinion Shopify is probably the best solution.