The old basic yellow eTrex has a limit of 6 characters. Anything more than that is truncated and duplicates (once truncated) are discarded.
So, you'll need to allocate 3 chars for the unique Id (possible to do it with 2 even on big projects, but 3 can be done more mindlessly I find)
2 was enough for the whole of LEL. Using letters alone 26*26 = 676 instructions. Letters plus numbers: 36*36=1296.
I just write the instructions into Bikely (or Bikehike) without the bits required to make it unqiue, download the route and run it through a script to rewrite the instruction unique bits automatically for me. It checks the number of points needed and uses the easiest system (i.e. numbers for routes with under 100 points, letters up to 676, letters+numbers above, etc). It'll even handle a few instructions that are 4 or 5 characters, it just makes sure all of the point names end up being unique.
I then have an underscore or hyphen to use up a 3rd character. That leaves 3 characters for the instruction, giving:-
SO, LT, RT, L, R, 3R, 4L, E1, E3, but also 3 characters for some instructions: LiR, SOX, STR, FIN, INF, etc.
Not a problem now as I use an Edge 705...