517 Not one Parnassus, nor one Helicon

Parnassus > (Mount Parnassus, chief seat of Apollo, god of song and
music, and leader of the Muses)
Helicon > (Mount Helicon, sacred to Apollo and his Muses)

518 Left for sweet Muses to be harboured, 519 But where you yourself have your brave mansion;

But > Except brave > splendid mansion > dwelling-place; mansion

520 There indeed dwell fair Graces many one:

Graces > (The handmaids of Venus, bestowers of beauty and charm)

521 And gentle nymphs, delights of learned wits,

nymphs > (Minor female divinities with whom the Greeks peopled all
parts of nature: the seas, springs, rivers, grottoes, trees,
mountains)
wits > minds

522 And in your person without paragon

paragon > comparison; competition