And just in case you're really really short of GPIO, you can use a 74595 shift register to feed the column and row selection, so you'd just need clock and data

And just in case you're really really short of GPIO, you can use a 74595 shift register to feed the column and row selection, so you'd just need clock and data outputs. You could also use 74251 8-to-1 multiplexer as that has an output enable, so you could use your remaining 2 outputs from the shift register to enable one of the two multiplexer chips. If you're not latching the outputs on the shift register, then you'd want a resistor between the two outputs so they don't fight. So then, the ultimate reduction in GPIO would be to 3 lines - clock, data, in. Obviously, for each of these, as you reduce the number of IO pins, you increase the time required to scan the matrix.