The numbers people quote in thousandths are real but they are usually answering a different question than the one being asked, which is why they are confusing.
What actually matters is the relationship between camber and depth of cut. If your camber is deeper than your shaving is thick, the corners cannot touch and you get no tracks. If your shaving is thicker than the camber, they will. So a very fine smoother needs very little camber, and the same iron in a plane set for a thicker cut would need more.
Which is why one specific number for a smoother is not really available, it depends on how fine you are working. What people mean when they say a couple of thousandths is roughly "more than a fine shaving and less than you can obviously see".
The practical consequence, and the reason I would not chase a figure: if you get tracks, add a little more camber or take a finer shaving. Those are the two knobs and either fixes it. Adjust and test rather than measuring.
One thing worth knowing that does need a number, though: whatever camber you put on, it has to be symmetrical, or the plane will cut deeper on one side and you will chase a taper across the board forever. Checking against a straight edge under a light is the whole quality control step and it takes five seconds.