Just one nation -- the one that returns from the Diaspora.
What nation do Preterists think it is????
Well, interpreting Scripture with Scripture one can only conclude it is the "holy nation" that Peter references. "But you are a chosen generation, a royal priesthood, a holy nation," (1 Peter 2:9). Not future tense. Present tense. The only "holy nation" at that time was the Church, what Paul in Ephesians 2 describes as the commonwealth made up of Jews and gentiles.
BTW, it’s not just preterist that interpret this correctly, but most if not all non-dispensationalists. Perhaps all non-hyper-dispensationalists, since I have never heard a dispensationalist try to argue that the "holy nation" of 1 Peter 2 is not the Church. That is a new one on me.