How to call an interrupt when any one of the 12 defined digital inputs go high?
You are kind of unlucky. The ATmega2560 MCU has 32 pins capable of
generating interrupts (counting the INTn and PCINTn, not sure all of
them are routed to board headers), but none of the pins you have chosen
is interrupt-capable.

I suggest you combine your 12 signals, using external logic, into a
single interrupt signal. You would then, in the ISR, sort out which
individual signal triggered the interrupt. A pair of 74HC4075 chips
should be enough to OR together the 12 inputs.