Welcome to the SeeMeCNC Community

I downloaded this http://www.thingiverse.com/thing:53886/#files
It is for my M2 printer. The walls on this part are too thin giving
the part too much flexibility so I figured I would fix the problem
by altering the number of perimeters under vertical shells. I set
the number of vertical shells to 20 just as an experiment and it
made no difference in the print, it still prints 2 walls. My nozzle
size is .35 and the normal wall size (1 perimeter) is around .4mm
20 of them should have been 8mm thick walls but as indicated
my setting of 20 was ignored. This is the latest version of Slic3r.
Does anyone have any idea why my settings were ignored?

I think the slicer settings aren't supposed to change the dimensions of the part, they just guide the slicer on how to build it. If you want more walls in the same amount of space, then your nozzle diam needs to go down or else they won't fit in the wall as defined in the stl.

I'd probably print with a tougher material or recreate the model to have thicker walls.

barnett wrote:I think the slicer settings aren't supposed to change the dimensions of the part, they just guide the slicer on how to build it. If you want more walls in the same amount of space, then your nozzle diam needs to go down or else they won't fit in the wall as defined in the stl.

I'd probably print with a tougher material or recreate the model to have thicker walls.

I think I understand now. The STL determines how wide the wall of the part is and that cannot be changed by the slicer.
It sounds as though the only way to make the wall thicker would be to edit the part within a Cad program.
Thanks for making it easier for me to understand.