How do I reduce the number of GPIO pins used for a CMOS camera?
Going by the pinout described here, you would need 5x 8-data lines, 2 config lines for each camera. The clock and synchronization signals (4), plus the power/reset signals (2) could be common to all of the devices. This would give 56 total signals required, plus power/ground to each of the devices.

However, I'm not sure you can receive all 5 sets of data correctly from the devices using GPIO pins.