As far as I remember, nobody has requested explicitly more MIDI tracks and I can’t recall Polyend a hard limitation on 8 MIDI channels (although it is hard to follow everything). I remember two wishes related to getting more wiggle room in the MIDI/Synth mode:
- Dedicated MIDI in/out channels for synths (welcoming votes)
- Dedicated Tracks for the Synth-Engines (declined)
I have expressed my opinion that 8 MIDI notes at the same time is more than enough, but who am I. 
Note that the limitation is 8 simultaneous MIDI notes, not 8 MIDI tracks, since each track can contain notes pointing to different MIDI channels.