Handwiring with Pro Micro - no. of pins

[haricane8133]

This is the exact keyboard layout that I absolutely prefer. I am trying to build this keyboard by hand from scratch. So I am not using the PCB that has come up with 68keys.

I built a plate to hold the switches using the DXF layout files provided, and I have also soldered the switches with the 17 columns and the 5 rows, as specified in the 68keys layout. I am looking forward to using the pro-micro and the provided QMK binary to flash directly

Now I have noticed that the number of I/O pins required for this is 22 (17 columns + 5 rows). Here is where I, while hand wiring, am blocked... The 68keys PCB seems to use the same Pro Micro as I am using, but the Pro Micro contains fewer pins for use.

It would be really helpful if you could provide some insights on how we are able to use the available pins of Pro Micro for 68keys. I would be happy to contribute a hand-wiring section in the documentation if I am successful.