Nephi surely knew nothing of the population sizes required for various political systems, but he knew the Lord, and knew that the Lord had prophesied that Nephi would lead his people. Nephi sees this people as the fulfillment of the prophecy.
Nephi's brothers had always feared the prophecy, partially because they saw it as usurping their birthright, and partially because they could see it becoming true as Nephi became a de facto leader. What is interesting from a spiritual perspective, and as an insight into Nephi's character, he apparently does not see the fulfillment of the prophecy until he actually becomes a leader by voice of his people.
It is this way with the fulfillment of many prophecies. They are fulfilled in quite natural ways, in the flow of the events of life and the world.