I don’t think I phrased it correctly. I mean in game to prevent having some 30+ individual skill scrolls the item itself uses its durability to know what skill it can currently teach.
Using random skills as an example, at 20 durabilty it teaches Sol while at 19 it teaches Adept, and is consumed after a single use either way. Which is entirely due to how the ASM (i think thats what its called) is set up.
I read the documentation here: The Skill System and You: Maximizing Your Usage of FE8's Most Prolific Bundle of Wizardry - #8 by Sme. Then experimented and after a few tries apparently got the skill scroll set up correctly. For the test, I gave a Fortune skill scroll (it has the highest skill ID value at FE) to Roy at the start of C20x:
Sent the Member Card to Supply. So Roy’s inventory looks like this:
And his skills list looks like this:
Ok I use the scroll:
And he learned Fortune:
So you’re right about the skill scrolls having durability mapped to the skills (at least the documentation says so). But, there’s no problem with teaching any skill due to any sort of cap on durability.