Set a universal limit for the max quantity, per product, that can be added to cart. Does not require customers to log in.
I don't have the time at the moment to add it with the proper option to disable it for those who do not want this feature. (There are those who do not prefer this feature. For example, if you want to limit sales to 3 T-shirts, regardless of T-shirt size.) I may add this feature in the future, when I get time. You are welcome to hack away on Github.