The conversions are listed by their land. Typically, there is a city that gives its name to the surrounding land. There is a city of Nephi in the land of Nephi. When verse 12 lists the land of Shemlon, it lists two particular cities, the city of Lemuel and the city of Shimnilom. That might suggest that there were other cities in the land that were not converted, but that is speculation. There is no real information here to understand much more than what is written.
What is important in this list is the culminating statement in verse 13. These cities were converted, and they “laid down the weapons of their rebellion, yea, all their weapons of war.” As noted in the previous verses, this will become an important feature of the story of the Anti-Nephi-Lehies.
Either Mormon or Alma, since the author is unclear, underscore that all these conversions were Lamanites. Mormon would have either added that line, or made sure to copy it, to make certain that his future readers, whom he assumed would be Lamanites, understood that they, too, were capable of faithful conversion and redemption.
This section of the story ends by noting that the Amalekites, who were apostate Nephites, were not among the converts. It is another theme Mormon emphasizes, that the worst and most implacable enemies were those who apostatized.