I think the word "semantics" is singular (just like e.g. "mathematics"), at least it is used that way (e.g. "... the semantics of ... "). I know its meaning at least in technical terms and I understand that often it makes sense to "treat it as singular" such as stated here. However, I definitively need the plural form and it feels like "semantices" sounds terrible.
I insist that "semantics" is countable in the context at hand. I use "a semantics" as a mathematical term that refers to basically a function that maps strings of symbols to mathematical objects. Those functions are a countable quantity (in the linguistic, not the mathematical meaning of the term).
I do understand that "semantics" is already in plural but this doesn't solve the problem at hand because, if what Jon Hanna says is correct, then it "doesnt work for something countable".