If I plan on using my iPhone & octatrack mk 1 connected by a USB midi interface is portable small and affordable as possible how many channels does it need to have for me to be able to use various apps on iPhone for adding effects like reverb and mixer etc.?
A standard MIDI port provides 16 MIDI channels (in one direction). But MIDI is only used to send/receive control informations like notes or control change messages and has nothing to do with audio (like you are implying by writing about adding effects).
If you want to use your iphone to process audio from an Octatrack you will need an audio interface.
Thanks .
I guess I was confused by the various labels on the interfaces like MiO 1x1 or 2x2 etc.
Is there a preferred cheap and small interface for octatrack & A4?
Maybe this will help:
https://www.elektronauts.com/t/ipad-audio-and-midi-to-ot/65652
If you want to route audio back into the iphone you need an audio interface as mentioned above.
In my opinion it’s worth considering whether you really want/need all the added hassle just to do stuff on the go, especially when using just the OT alone is often more than enough 
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You know what klonk,
I think your probably right about whether the extra functionality to hassle ratio is a worthy investment.
It’s not the on the go thing that I’m after as much as the affordability of using iPhone audio effect apps and also finding a way to process each track separately or really just only fx the ones needed as I have the older models if A4 and ot