I made a very similar question some time ago, and it appears that the most recorded piano piece is actually "FUR ELISE" (more than 300 commercial recordings available, as i seem to recall). This was quite surprising to me, because i had the idea that being a so overplayed piece by early students, it wasnt so often recorded...but, after all, even the great Brendel recorded his version of it !!! By the way, "Moonlight" sonata was up there too.
I made a very similar question some time ago, and it appears that the most recorded piano piece is actually "FUR ELISE" (more than 300 commercial recordings available, as i seem to recall).