In "the seventeenth year, in the latter end of the year" (3 Nephi 3:22) a proclamation by the governor called all the people together to a place which was appointed in an attempt to overcome the Gadianton robbers.:
"And the land which was appointed was the land of Zarahemla, and the land which was between the land Zarahemla and the land Bountiful, yea, to the line which was between the land Bountiful and the land Desolation" (3 Nephi 3:23).
And there were a great many thousand people who were called Nephites, who did gather themselves together in this land. Now Lachoneus did cause that they should gather themselves together in the land southward, because of the great curse which was upon the land northward.
At least by the "latter end of the eighteenth year" (Alma 4:1) when the robbers began to come down, the Nephites had gathered ("they did dwell in one land, and in one body"--Alma 3:24), they had fortified themselves (v. 25), and they had made weapons of war of every kind (v. 26), and they had sufficient numbers that they were able to overwhelm "tens of thousands" of the opposing army (Alma 4:21).
One has to wonder as to the limits this account places on the total land areas occupied by the Nephites. In other words, the total land areas could not have been so large nor so far distant that many thousands of Nephite inhabitants could not be accommodated centrally in an area of limited dimensions within a short period of time. Even with excellent communication facilities, the short period of time of less than a year puts limits on the extent of the territory.