We have recently run into a problem. A customer was able to order 4 of a product, even though we only had 2 in stock. We do not have the 'Continue selling when out of stock' checked, and we do have the 'Track quantity' checked. Is there anything we can do so that once the customer goes to their cart that will show them that only a certain quantity is available and they have the option of 'continue shopping', instead of them being charged for a product that we can not deliver and must partially refund their order or make a substitution?
It seems that 'Continue selling when out of stock' is the only place where you can limit customer orders in Shopify. I do a quick test and it works on my case; you should run a double check on your products to ensure that your stocks are up to date.
Other than that, limiting the number of items a customer can add to a cart is a feature that can be added to your store through an App Integration. There are a couple of apps that let you specify minimum orders as well as maximum orders by price or item quantity limits, you can see them below:
Hopefully, this might help!