I graduated a couple years ago, but I'll answer anyway. I'm not sure I'd bother getting spare loupes given the quality of the cheap loupes. If you start with 2.5x mag, there is a reasonable chance you'll bump up. In that case, the 2.5x loupes make excellent backup loupes.
The advice I'm more confident about is that the cheap Amazon loupes can only be used at 2.5x mag.
The 3.5x, 4x, 4.5x ect all focus terribly. If you do manage to get the image right at these higher magnifications, you need to readjust constantly.
Tl;dr Don't get the cheap Amazon/Ebay loupes in high mag.