For this subject the manual is in some places actually refering to something that's not in Modular. Like section "3.2.11.5 Unisson" and section "3.2.11.6 Maximum polyphony".
Very confusing indeed.
But if you go to the section "3.6.4 The play modes", then there a an image showing the playmode parameters.
You have a switch and a display above the "Poly" label where you can set the number of voices. If you click + hold + drag up or down you can set the number voices between 1 and 32.
When you set the playmode switch to "Poly", then you can play from 1 note/ voice and up to the number set in the display.
When you set the playmode switch to "Unison", then you can only play one note, but each note have as many voices as you set in the display.
Now what are voices.
A Voice can be used by multiple OSC modules, that can be tuned seperately, and thus for example can be heard in different tunings. But it's still only one voice. This is how a Monophonic synth or playmode works.
If a Synth is Polyphonic or when in Polyphonic playmode, then each OSC module actually have as many OSCs as it has Polyphonic voices. In Arturias Modular that's up to 32 voices/ OSCs for each of the 9 OSC modules. That's for example called 32 voice Polyphony if set to 32 voices. This means you can play up to 32 notes at a time on a keyboard for example, and hear all those notes at the same time.
When in Unison mode all those up to 9 OSC modules x 32 voices (That's 288 OSCs) are/ can be played by one note. You can only play one note at a time like in Mono mode.
The up to 32 voices you set is the display can be detuned using the "Voice Detune" knob. And each of the 3x3 OSC modules inside each Driver section (that's 9 OSC modules) can each be tuned seperately as well.
Hope this short explanation helps.
I wish Modular had the possibility to Pan Unison Voices to spread the voice over the stereo field. This require a "Stereo mode" for Modular. So perhaps it's tricky or not possible or reasonable to do in Arturias Modular, for one reason or another. But it would make Unison huge, and also add to Polyphonic mode for that matter.