I still think you are wrong about that. To home each motor you will need to have them turn independantly as well as together, and have a home sensor for each disk.
3 pins for 74hc595/tpic6b595, 2 pins for mcp23017/pcf8575 etc.
With 74hc585/tpic6b595, they can be daisy-chained, so data goes from Arduino to first chip, data out from first chip goes to second chip etc. Clock and latch signals are shared by all chips.
With mcp23017, pcf8575 etc, the data and clock signals are shared by all chips, and each chip is given a different "address" on the i2c bus by wiring their "An" pins in different ways.