INT0,1,2,3 are physical pins 43,44,45,46 on the ATMega2560. D21,20,19,18 on the Mega2560 schematic.
But reference says:
Most Arduino boards have two external interrupts: numbers 0 (on digital pin 2) and 1 (on digital pin 3). The Arduino Mega has an additional four: numbers 2 (pin 21), 3 (pin 20), 4 (pin 19), and 5 (pin 18).
So leaving physical pin numbers out of it and leaving Atmel pin names out of it also to void confusion, and just using arduino shield connector pin numbers and the proper user interrupt name used in sketches, The Arduino reference above says for a mega board, six total user interrupts supported on the following arduino pins:
sketch interrupt number arduino pin number
I've only used interrupts on arduion pins 2 and 3 on my mega board, but I assumed the pin numbers in the arduino reference to be accurate, that there are a total of 6 available user interrupt pins on a mega and they are at pins 2,3, and 18-21?