Rashi (Shemos 26:6,26) counts out the number of poles for the chazer on each side in parshas Terumah. He seems to ignore what is called the Fencepost Problem. The same problem exists for the 20 amah enclosure on the East side. Do any of the commentators address this?
If I understand correctly, you are referring to the brichim, the horizontal poles that were slipped through rings on the boards -- and through holes within the boards -- to hold them in place. The math is straightforward: There were 20 boards of 1.5 amos on the north and south, thus requiring the poles to span 30 amos (the top and bottom poles were 2 x 15 amos, and the middle pole 1 x 30 amos). On the west was 8 boards of 1.5 amos, which required the poles to span 12 amos (again, top and bottom 2 x 6, middle 1 x 12).
Now, the 8 boards on the west overlapped one amah of the ones on the north and south. But evidently the poles on the N and S sides did not extend over the thickness of the western boards. So the math works out perfectly.