The article mentions 10 Memento Keys (5 golden, 5 iron-y) and four extra Keys that the game logs as Mementos upon pickup.
However, there are 15 Keys in total - the aforementioned 10 Memento ones, the 4 extra ones, and an additional Memento Key that is not shown in-article with its Note.
This Key is for the Memento Cache in Desolation Point. The article even mentions that there is a Key found along with the Note, but only displays a Note icon.
