'There be none of Beauty's daughters' by Lord Byron: This poem seems to have been set to music by over 110 composers, including Parry, Moscheles, Stanford, Quilter, Tovey, Fanny Mendelssohn, Armstrong Gibbs, Wolf, Castelnuovo-Tedesco and Holbrook. Is this a record for one piece of literature/poetry to be set. I wonder if anyone has done a thesis on all the settings, should make interesting reading.
I would have thought the answer (not counting pre-Classical texts) would be something by Goethe or "Ossian" (or Stevenson? hrm... maybe a few other guesses come to mind... Tagore has received a whole lot of modern settings... ... anyway) perhaps, but (using as a partial reference recmusic.org/lieder, as I am guessing you may also be doing? Excellent site, in my opinion) there seems offhand to be no one Goethe text (at a quick look) to have attracted quite so many settings- hrm...! (though one may wish to supplement that site with e.g. backtracking work-from/on-IMSLP- using the Librettist category for a writer on that site, and backtracking to see how many songs seem to belong to the same text- etc.)