Are you going to use any other interface? Serial, so you can talk to a PC? I2C so you can display on an LCD? If not, it looks like 51 is the limit: 54 digital I/O pins minus SCLK, MOSI, and MISO. You might need drivers on SCLK and MOSI to drive the 51 inputs from one output.
You could also use your choice of output expansion (shift registers, port expanders) to get even more SS pins on an UNO or Nano. Eight shift registers will use three pins of an UNO or Nano to get 64 output pins.