In general, the maximum number of units available for purchase of any one product is 999 units, since we do not hold larger amounts of stock for orders over that number.
On the other hand, there is a maximum of litres of water available for a single order. This limit is 120 litres in total. If that limit to the amount of water is reached, you cannot keep adding water to your cart and the system warns you automatically.
Also, some products that are prepared by hand can have a limit per purchase. An example of this case, would be the knife-cut ham.