The polyphony for these instruments is 24 Partials - if every sound used only
one Partial the instrument would be able to play back 24 notes. However, 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. 18 notes is probably a fairly accurate number
of notes that it can play back before the unit will run out and begin stealing
Partials from other parts.
Also, most of the drums are 1 Partial but this also counts toward the polyphony
total, and complex drum patterns can use up a lot of notes, especially when
using Cymbals and Hi-Hats - due to the long decay time associated with these