How do I run more than 2 circuits in conduit?
You can mark the white or gray neutral wires with colored tape. Unlike marking a switch loop wire in cable (where it changes white from neutral to hot), this does not change the function but is simply an identifier. Real simple:

Black + White or Black + White-with-black-tape
Red + White-with-red-tape
Yellow + White-with-yellow-tape
Blue + White-with-blue-tape

Of course, the colors don't have to match like that, but that would make it super easy to keep track of things.
With only two different circuits, yes white &amp; gray as two different neutrals makes sense. Once you get to taping things, it is arguably easier to use white for all neutrals instead of having two different colors, but that depends on how much wiring you do.