Numbering amino acid residues
This question is a bit hard to answer for me without more context.
However, it is clear that the n refers to the tyrosine which is highlighted in the figure as well (especially the interaction of its phenol). The other positions (n+1, n-1) are shown relative to the tyrosine.
I'm unsure about the role of tyrosine in the primary structure of proteins, but maybe this is more clearly explained where you found the figure. I'm also unsure if the glutamic acid is terminal.