1) None
2) eight

Ok, so by "sounds" you mean 8 elements per part. By "elements" you mean parameters that change the sound, or do you mean you can add additional instruments I.e. Violin, cello, etc.? And yes I'm reading the manual but I need a synth for dummies book as this is my first one.

Sorry, it can be very confusing especially because there are so many ways to accomplish a goal. There are truly more ways to accomplish things than you can possibly imagine particularly with a synth.

If you ask the question, "How can I create a piano / bass split?"
The question would be best answered with another question: do you want to build from scratch, do you want to use existing programs and combine them, are we talking easy or are we talking advanced?

The newbie might opt to take two programs already programmed by the experts and combine them in a single program. A legitimate way to work it.
The advanced programmer might opt to do it the most economical way possible... Depending on what else they are setting up to do.

The newbie might wind up using the majority of available oscillators (64) to accomplish the task.
The advanced programmer might just use the minimum number of oscillators to accomplish the task (2).
Probably somewhere in between would be the best way... The point being there is no *one way* to accomplish a piano/bass split.

Let's break it down:
A PART (the fundamental playable entity in Montage) can be made up of as few as 1 and as many as 8 oscillators. "oscillator" being a sound source.
If the PART is an AWM2 sample Part, there can be as many as 8 oscillators, called Elements. An Element is a collection of parameters that can be a complete instrument by itself.

Find, recall and play an AWM2 Performance called "Ens Mix". This is a full orchestra sound made from just a single AWM2 Element. Of course, an Element can be used in combination with other Element to build a sound. So it is totally possible that each of the Elements in a Part could be an entirely different instrument sound. However, mostly you will find that at maximum one, two or three instrument sounds, at the most, are made from the 8 possible Elements.

By using an Element to just be a portion (one among several building blocks) of the whole sound, you can give the playability of that sound more controllable nuance. Having a piano with 18 Elements means you can program it with great detail and nuance.

But it is totally possible to build the piano/bass split with just two Elements within a single Part.
And on the other hand you might use a four Part piano and split that with a two Part bass, using twenty or more Elements... The additional Elements could give the bass more dimension and the piano more nuance.

The playable Performance in Montage can be from one to easily 64 oscillators (even more if you include a drum kit Part). Each oscillator could be a complete instrument or used as a component in building an instrument. We can only recommend that you take your time, learn the fundamentals of sound building. Many users make the mistake of finding the biggest possible sound and then attempt to combine it (inappropriately) either another biggest possible sound, and are shocked that they've run out of resources.

We realize there is nothing you can do with those folks, except watch them self destruct with the technology. Understanding the reasons for programming decisions can help you when attempting to create your own Performances. Knowing whether or not an Element or component actually serves your ultimate goal is important. Calling up and building a sound using the "CFX Concert" piano and then never playing any velocities below 90 is, frankly, a waste of oscillators... A piano with so much detail in the soft and medium strike area should be used in a situation where they can be exploited through expressive play, not in a situation where you are banging out a forte only performance...if you get my meaning.

On the FM-X side there are also 8 oscillators per Part, but bring a pure synthesis engine (no samples) the roles of the oscillators have an entirely different slant. They are used to interact with each other in order to generate a more complex timbre, while AWM2 oscillators are digital recordings of actual instruments, sounds, sound effects, etc, the FM-X oscillator, called an Operator, generates a sine wave or a range of spectral forms that can be used to created more and more complex waveforms.

So just *when* do you decide to make that piano bass split in a single Part, and when would you use two or more Parts to make it? That's a question the programmer must ask and answer for their situation. So a question like yours, "how many sounds per Part" has many answers.

If a Drum Kit Part is selected - it has 73 Elements, one per Key. A different drum or percussion instrument or articulation on each of the 73 Keys.
But I answered 8 because although a correct answer could easily be 73, it would come with an asterisk that explained what a drum kit is... take your time.

For those who feel they need 16 Parts, I highly suggest you learn what is possible with just 1 Part before demanding something that may not even be necessary for what you want to accomplish. Thanks for the question.

I have a suggestion. Maybe Yamaha can offer a set of single part performance pack that only focuses on this issue. For example: Single part performances that showcase 2 or 3 or 4 controlled instruments within that one part performance. Figuring out how to control them all within "performance mode" would be tricky unless that was also put together in the pack and thoroughly explained.

If you simply stay in "part mode" then you can use all 16 parts effectively as they are but only play one at a time. If anyone has constructed such a compilation as mentioned above maybe you could share your file with instructions as a learning tool.

I wish there was a "multi-part" mode that was not the same as performance mode where you could at least select several parts in a layer with something like using the shift key and picking 2 or 3 parts at once. I forget if part mode, before selecting a single part, cancels all the other parts. If in "mute mode", subtractively you silence all unwanted parts but that takes some effort.

I accomplish this work-around task using the MIDI loop cable trick while in performance mode where my additional layers are on the channels 9-16 and my primary parts stay on channels 1-8. In "mixer" mode I can turn on/off (mute) the parts I don't need in real time.