Ethnostate is an ambiguous word. Does it mean ethnically homogenous? Israel is certainly not that. Does it mean, essentially, a nation-state whose "national group" is an ethnicity? Israel is that - but so is much of Europe and some parts of Asia.

How many of those have declared themselves to be the Nation-State of the X people? (at least after the 1940s)

Many former Soviet and former Yugoslavia nations did this.

I ran the question by AI, and it seems to think that somewhere between 30-50 since WWII have done so explicitly.

Seems fair to call those ethnostates as well.

Edit: I asked ChatGPT "How many nations have declared themselves to be the Nation-State of a people?" earlier and got the same answer as you. I asked again just now and got "Israel is the only nation in the world that has legally declared itself the “Nation‑State of the Jewish People.”" [I didn't specify Jewish]

Can't rely on AI.

You should consider that AI is often wrong and stop using it as a source of truth.