Ok- so ive never had to use a crutch- but dosn't it go under the arm on the side with your bad leg?
Nop~ It goes under the arm of the side of the good leg. Last week people corrected the author. So in the post after this chapter the author said sorry and thanks for the correction.
You can google it or even better ask someone with better info but is not exactly a "repleacement of the leg" but more like a help for balance without alter the natural movement in walking, and it helps so that the half of the weight goes to the crutch.
But doesn't that apply to cases where you are actually using your injured leg, thus needing the support for balance? Here she isn't using her injured leg at all (in which case I'd imagine she would benefit more from a pair of crutches rather than just the one?).
If the case is you can't use the foot at all, yes, a pair of them is needed. I don't really know for sure if Koga is able to touch with her foot tho; yes, in all panels we see Koga with the foot up, but since it says "terrible sprain"....honestly this is better analysed if it was a real person hahaha
For example: I had a sprain in which I couldn't walk but could ""touch"" the floor (tho the pain was awful) and I needed a support to walk, tho because... me hahah (Im a beast, not in the good way(?) I chose a broom and used it like a crutch hahaha And it kinda worked (note: people use a crutch. I end up with others pains later)
So basically for the case of one crutch, I think it really depends to the person and injury that is talked in the moment and the specialist