Hi @matteolavitnicora. I found some useful information on the subject from one of the developers of the "Arduino Mbed OS Portenta Boards" platform here:
https://github.com/arduino/ArduinoCore-mbed/issues/789#issuecomment-1840246454
the Giga allows a maximum of 16 interrupts at the same time, and the coexistence is based on STM32 peculiar way of handling them (like, PA_0 will share the interrupt with PB_0 , PC_0 and so on). If you instantiate 2 interrupts on the same line the board will "crash" (report it as invaliud).
To associate pin name and pin number you can use this table ArduinoCore-mbed/variants/GIGA/variant.cpp at main · arduino/ArduinoCore-mbed · GitHub
That is about the GIGA, but should be relevant to the Portenta H7 as well. The Portenta H7 equivalent to the link in that comment is here:
https://github.com/arduino/ArduinoCore-mbed/blob/4.1.1/variants/PORTENTA_H7_M7/variant.cpp#L22-L225