There may, in fact, be at least 20 unique characters among the Zonai letter shapes! You can find several of these alternate characters on the various classes of Zonaite weapons (both swords and spears), from the basic Zonaite set all the way up to Mighty Zonaite set.
For comparison, Wilds era Hylian (first introduced in A Link Between Worlds) is an English cipher font which has 21 unique characters; with five letters acting as two different readings each (i.e., D and G, E and W, F and R, J and T, O and Z).
As discovered from the game's files and from careful observations, there are at least fourteen (14) primary Zonai letters that make up the majority of their written works. This isn't including the variant writing system seen in their magic, e.g., in the Shrines of Light or in Rauru's seal on the Demon King. That's a story for another day~
So, upon examining the Zonaite weaponry, it appears that some of these basic runes are compound characters! Since their appearance is much like Chinese hanzi or Japanese kanji, we will refer to these compound components as "radicals."
Interesting note, many of the radicals included in other characters tend to display one or both of the following traits: either an additional stroke or dot, particularly in their centre, or two added strokes extending from their sides—I call these "fins".
Since some of the basic Zonai letters also include these traits, it could be that some of these letters are compounds of even more primary forms, cobbled together as radicals in a single character. Isn't that neat?