How many digital pins can I drive with Arduino DUE?

I'm testing a chip that our lab built, and I need to drive a lot of IO pins (I think I need at least 55 pins).

I wanted to use an Arduino Due to drive these IO pins, but I'm not sure which pins I can actually use as digital IO pins. So I'm looking at the sweet pin diagram that shows all of the many names for every pin on the Arduino due. If I count the number of pins that have a digital pin number in grey next to them, I get 72 potential pins.

There are a few pins that appear to be marked as "unsupported" like RXD2 and TXD2, but I'm guessing that just means the arduino libraries don't support using them as a serial port yet (but I can still use them as a digital IO). Is that correct?

I would like to avoid using pins that are also used for other stuff, particularly the tx and rx pins that are used for programming the board, because I don't want those signals propagating to the chip I'm testing. Are there other pins like that which I should avoid?

Do any of the other pins need to be handle specially in order to be used as a plain old digital IO pin?

One more question. When I write an arduino program, which of the labels in the diagram should I use to refer to the pins? I know there are usually macros defined for these types of things in the header files. For example, if I want to set RXD2/SPI-CS2/D52/B.21/92 as an output, would I do pinMode(D52, output), or pinMode(92, output), or something else?

Thanks in advance to whoever can answer these questions!

byte pin52 = 52; // or some meaningful name

pinMode (pin52, OUTPUT);

digitalWrite (pin52, HIGH);

OK so basically all of the DXX, sans the D part. Thanks. That answers the second question.

Can anyone else answer the other questions?

If they're brought out to a header pin you can use them as digital IO. Probably avoid D0, D1 for the serial port.

Read the uC datasheet http://www.atmel.com/Images/Atmel-11057-32-bit-Cortex-M3-Microcontroller-SAM3X-SAM3A_Datasheet.pdf to confirm if the analog inputs or DAC outputs support digital IO also.

Or just try them, as digital 54-70, see if code will compile, and try them with a scope, a meter, or an LED/resistor.