Using the most recent data available, nmsc determines an allocation of Semifinalists for each of the 50 states, based on the state’s percentage of the national total of graduating high school seniors. For example, the Semifinalist allocation for a state that enrolls approximately 2 percent of the nation’s high school graduates would be about 320 (2 percent of the 16,000 Semifinalists).
(bolding my own)
So, the amount of NM Semifinalists for each state is determined by the number of high school graduates, not test-takers.