As I understand it, the settings are based on the max thickness so technically the wall calculation is as follows:
max thickness/nozzle = wall count : max thick 3.2/ line width 0.4 = 8walls
Some people will add a few more walls for “safety” which is why I guess they just put in 999 because it’s easiest. The printer won’t actually print 999 walls.
The idea is to have a completely solid print.
No matter what method you use it should print correctly.
This explains it better than I can and their calculation may be different but they all work: Slicer Settings for Lithophanes • Tweaking to Perfection • ItsLitho