The mastery profile screens show there are 755 cards that can be mastered. However as far as I can tell there are only 727 cards available in the game.
I notice that the difference between 727 and 755 is almost exactly the # of Transform cards in the game (26). And there may be a Transform card (Skin Invasion) that wasn’t released due to issues, that would account for the final 2 card entries.
I suspect either there are a lot of cards in the DB that haven’t been revealed yet, or the card mastery list is double-counting Transform/Transformed cards.