I noticed that this voice doesn't have the keyboard control option available, I don't know if that has something to do with the problem, it seems like I can only mix 8 voices in the scenes options.

You can play eight Parts at a time. You can link any Parts 1-8 by activating the KBD CTRL icon. That icon is available only for Parts 1-8...
KBD CTRL = Keyboard Control and that is what it means, literally.

Is this the way it is supposed to be, or am I doing something wrong ? How can I resolve this ?

You haven’t given us enough info to make suggestions... (but I know the song well and have sequenced it myself...).

You say you are performing it live, so you are probably not manually playing the Drums (I’m guessing)... beyond that we can only guess what you hope to play yourself and what can be automated and played by MODX. The sequencer, or one of the eight Arpeggiators can be used to play Parts you are not actually playing yourself.

Let us know.

I am trying to more or less have this setup like it shows in this video: https://www.youtube.com/watch?v=sWLAeUawc_QI

For that I have three voices mixed in Scene One, Three voices split in scene two, one voice in Scene Three and one voice in Scene Four. I wanted to put a second voice in scene three but now that I see that I can only use eight parts maximum, what I'm going to do is sacrifice one of the voices in scene one so I can do the other stuff in scene three which it is a little bit more important.

The video shows someone playing along with the record. (?)

What I was asking you was, what instruments you are going to have the MODX play? If any...
And which instruments are you going to actually play yourself?

For example, the drums, bass line, and clavi parts could be played by the MODX’s sequencer... while you play whatever parts you want on top. Or perhaps I’m misunderstanding your goal... are going to play along with the record (like in the video)?
Do you not want to play with the sequencer?

Let us know.

The first scene is a mixed of a string and a synth which I would most likely play live, scene two is sort of what you can listen from 0:11 to 0:47 which are three split voices, then I would play the next part after that. As for the Clavi yes I can sequence it as well as the drums and bass. At this point I don't know if I would play this song with my band (depending...) or just me by my self. If we decide it to play it on the band(after Corona Virus is gone) I would like to sequence some of the parts and have the drummer play on top of it, however since the MODX doesn't have assignable outputs for a drum click I would most likely do it through MIDI but that would be a separate issue that I will very soon ask you about in a separate topic.

Just FYI: It’s the MONTAGE that allows the click to be routed to the Assignable Outputs

As far as the MODX is concerned place the Parts that you want the Sequencer to play in Parts 9-16, and those you wish to directly control via the MODX keys in Parts 1-8 using KBD CTRL to link them.

If you look into the Parts you want to combine for this song, you may discover unused Elements within some of the Performance Parts that you have selected. This means it may be possible to layer two instrument sounds within a single Part. For example, if you find that one of the Parts only use four of its potential 8 Elements, you can build what you need with the other four Elements.

This type of sound construction requires you analyze what each Part is doing, you may find that because many Performance Parts use some of the Elements for an articulation that you may not be using... for example, you might like a string sound that has several of the Elements in reserve for a pizzicato gesture that you’re not interested in using in this application, or a brass ensemble that has “doits” and “falloffs” that sit waiting for an Assign Switch. You can reprogram these Elements to build your own instrument.

Take a look at the Single Part Performance “Piano & Strings”. The acoustic piano is Elements 1-4, the strings are Elements 5-8. And since routing to the Dual Insertion Effects is done on a per Element basis, you can create a two instrument Single Part with out much compromise in how they work, play or sound. Just a thought.

I find the Motif ES Voice Data (which predated the 8 Element structure introduced in the Motif XS) is a wonderful resource for building custom MODX/MONTAGE sounds because they all only use four Elements. Additionally, they only use 2 of the 8 possible Assign Knobs, and they come free of any Super Knob assignments. So you get the fundamental instrument with plenty of room for customization!