Reply To: Custom numbered list (21 letter alphabet)

That’s a fascinating question! I don’t know of any way to do that “built-in” but I wonder if you change the language to Italian if it changes that?
https://creativepro.com/change-ui-language-indesign-cs6-cc.php

The only other way I can think of is to create a custom font (using any font tool, such as IndyFont or Fontlab or something) that skips those letters. For example, if there is no “J” and “K” then you’d put the “M” in the place of the “J” and so on.