The display and touchscreen uses the hardware I2C Pins (SDA & SCL), SPI pins (SCK, MOSI, MISO, CE0) as well as GPIO #25 and #24. All other GPIO are unused and you can still share the I2C pins with sensors, LED drivers, etc. Since we had a tiny bit of space, there's 4 slim tactile switches wired to four GPIOs, that you can use if you want to make a basic user interface. For example, you can use one as a power on/off button.