This project demonstrates the creation of a "Rule of 99" inventory system, you can read more here.
The map displays a boat and a couple of keys that are automatically added to inventory when the hero collides with them. When an item is collided with, the dictionary object, used to maintain inventory, adds the new key, if needed, and then increments the value. The maximum allowed per item count is controlled by a global variable and is set to 99.
To display the inventory screen you can either click the hero portrait in the upper left corner or press the Z key.