punchy_channels.ino

// Channel can be a number from 0 to 15. Typically displayed as 1 to 16.
byte channel = 0;

// ...

void setup() {
  // ...
  pinMode(18, INPUT_PULLUP);
  pinMode(19, INPUT_PULLUP);
}

void loop() {
  if (digitalRead(18) == LOW) { channel = 0 }
  if (digitalRead(19) == LOW) { channel = 1 }
  // ...
}

void noteOn(byte pitch) {
  MidiUSB.sendMIDI({0x09, 0x90 | channel, pitch, 127});
  MidiUSB.flush();
}

void noteOff(byte pitch) {
  MidiUSB.sendMIDI({0x08, 0x80 | channel, pitch, 0});
  MidiUSB.flush();
}