I agree that generally the "masterpiece" items are the ones that break away from what is typical in the era.  

It can break away by starting a new genre, it can break away by offering qualities far beyond the current state of the art, it can break away by offering a substantially different element that has major appeal.  

I disagree about having multiple masterpieces. Creating one product that breaks the mold is hard.  The odds of creating a second are quite low, it is rare, but it could still happen.

John Carmack is one such example. He broke the mold with the Commander Keen series with major innovation the 2D side scroller for the era, using the hardware as never seen before and impressing millions.  He broke the mold again introducing the first person shooter in Wolfenstein 3D which was revolutionary for the era with advanced graphics. And again he astonished the world with Doom which was revolutionary for the era and pushed systems to their limits, and slightly less of a defining masterpiece with Quake which was amazing and introduced many multiplayer online game modes but was not quite revolutionary for the era. After radically shifting gears, he made defining contributions to this era's VR landscape and the Oculus VR system.

Richard Grariot was another, with the first major electronic D&D game in 1977, then with the genre-creating Ultima series, and to a lesser extent his leading role with City of Heros/Villains. Either 2 or 3 depending on how you count it.