Of all the portraits that Paul Cézanne painted during his lifetime (1839-1906), his most frequent subject was himself. Second came his wife, Marie-Hortense Fiquet (1850-1922). The exhibition “Madame Cézanne,” opening Wednesday and running through March 15 at the Metropolitan Museum of Art, will present 23 of the 29 images of her that Cézanne is known to have made. They don’t give much away. Far from idealized as a woman or a beloved mate, she usually appears stiffly reserved, a dignified enigma. In fact, not a lot is known about her.