We are getting into the finer points of the definitions. You've played single Part programs that are multiple instrument sounds. Think back to Motif XF, "Piano & Strings", "Piano and Ooh Choir", these have several Elements to make up the piano, and several Elements that make up the other sound.

Well, in fact, it only requires 1 Element to create a full instrument sound. While talking about XF remember "Ens Mix"? ...this Single Part program is the full string orchestra in a single Stereo Element sound. Because the program architecture (AWM2) allows each Element its own complete path, its own tuning, its own filter, amplifier, its own routing to effects, a single Element can stand on its own as a complete instrument sound.

There are usually single Element instruments sounds in every musical category. So there can be 8 Elements in a Part. Each Element could be a separate instrument. Obviously, if you made each Element a separate instrument sound, this would be compromising some what its autonomy - because you couldn't isolate it completely in its own Insert, but the technical answer to 'how many instrument sounds in a single Part', is eight.

Do you follow what I mean? It's a matter of theoretical maximum traded off against practical use. Just because you can put eight different instruments in a single Part, doesn't mean you should... But you could, should you find a use for it. You wind up giving up some basic things like muting, and absolute individual controller assignments, etc. (it just gets tricker to evoke separate control).

In the "Programming Basics" series we show how with just two Elements you can have two entirely different drawbar setting organ sounds (one is your typical jazz organ setting, the other is typical gospel setting), and we show how to morph between them. You could have eight different single Element organs and use motion control to morph settings.

You could have eight different single Element instruments and have the Montage randomly play each, or cycle through them one after the other. Or create scenarios where Elements are paired in groups... Etc., etc., etc.
Remember a Single Element references a Waveform. A Waveform can have 128 stereo samples, can be not only a multi-sampled, but multi-velocity entity, all by itself!