In the RAMPS v1.4 board, it seems like the X, Y, and Z motors all use at least one analog pin, while the extruder motors are entirely digital. See just the top left block of this photo:
Is there any advantage to using analog vs. digital pins in the motors?
As far as I can tell the traditional thermistors have to be have an Analog pin input because of the way they are read and processed. The MOSFETs for the heated bed and hot ends (and fans) should be digital because of the way transistors work. The end-stops are also digital because they are on-off switches.
But besides that, I can't tell if there is any other requirements or advantages to using some pins in analog / digital, especially for the motors which have seemingly contradictory pin assignments.
There are also three digital pins I don't recognize: MISO, MOSI, and SCK.
Right now I'm looking to add 2 new motors using 8 total pins (6 Digital pins, 2 for ground). It seems to me I could use any combination of the D pin available in the image: