EULER bus question...

[MarkHaysHarris777]

I'm trying to figure out how many GPIO pins I can use on the EULER bus, from the schematic I see some which look like they may be usable as GPIO so long as I don't need their 'primary' functions....

GPIO (general purpose input output) can be used for anything;  this is completely arbitrary.  As Martinayotte has said, you may need to disable the alternate functions in the dts|dtb, and you may need to disable a driver or two;

... I do not follow that plan.  If you need more gpio pins, expand the bus with serial to parallel arrangements ( you can control any number of pins [eight per 74hc595] with five lines )  data, shift clock, stor clock, OE, and MR.  Cascade as many 595 chips as you need.  Look into SPI;  however, SPI does require disabling uart(3) and uart(4). (both on the euler bus)

The secondary advantage of expanding the bus with shift registers ( SPI or i2c ) is that the there is some buffering between your pine board and your project.  Its fine for experimenting to go directly to the gpio pins (LED labs and the like) but real projects should have a well designed interface ; particularly where inductive loads are employed. 

gpio concept is the same for most SoC(s) including the pyboard, raspberry pi, arduino, PineA64, etc... arrangement and use is completely arbitrary depending upon needs and your own imagination.