When Paul turns to the Gentiles in Antioch after being rejected by the Jews, Luke reports –
When the Gentiles heard this, they were very glad and thanked the Lord for his message; and all who were chosen for eternal life became believers. – Acts 13:48
This is a reference not to nations but to the group of Gentile individuals at the synagogue who heard Paul.
You know that when you were still pagans, you were led astray and swept along in worshiping speechless idols. 1 Corinthians 12:2
In the verse above, “you” refers to the individual converts at Corinth.
Then he destroyed seven nations (ethne)in Canaan and gave their land to Israel as an inheritance. – Acts 13:19
And for three and a half days, all peoples, tribes, languages, and nations (ethnon) will stare at their bodies. No one will be allowed to bury them. – Revelation 11:9
In the two verses above, it is clear that “nations” refers to an ethnic grouping, not just to Gentile individuals.
We cannot yet be certain which meaning is intended in Matthew 28:19. We cannot yet answer the question whether the task of missions is merely reaching as many individuals as possible or reaching all the people groups of the world.