The way to do this, if you want to avoid MIDI (and you can certainly biuld your own MIDI devices) is probably to use piezo sensors, connected to the Axo through a shift register. I don’t know for certain if there’s a shift register object, but it’s definitely possible. You then extend the pulses of the piezos using a capacitor and resistor to make sure the shift register can see then as it cycles through.
You can read about shift registers here: https://learn.sparkfun.com/tutorials/shift-registers/all
However, what I’d do is build a MIDI device. Without any fancy footwork, an Arduino can handle 11 or so inputs, 13 with neglible footwork (i.e. making sure pins 0 and 1 are disconnected when you flash the firmware, then reconnect them). Then, getting it to speak MIDI is really easy: it’s just a library that you call in your code.
If you need more inputs, you can use two Arduini (use a Nano — you can get them for less than $3 apiece), either both as separate MIDI devices or just having one speak to the other with PWM.