The polyphony is actually considered 32 Partials of polyphony and if every
sound used only one partial it would be able to play back 32 notes.
Unfortunately, many instrument sounds use 2 layered Partials per note. The more
complex the sound, the more likely it is that it is layering 2 Partials.
The number of partials per Tone is documented in the back of your manual on the
chart that listed all the sounds. 22 notes is probably a fairly accurate number
of notes that it can play back before it runs out and begins stealing Partials
from other parts.
Also, most of the drums are 1 Partial but that also counts toward the polyphony
total, and complicated drum patterns can use up a lot of notes, especially when
you use Cymbals and Hi-Hats.