At what point will they start to notice people are missing?
10 million?
The question is, how many people do you know? And are they evenly spread throughout the country? I posit that even one in 5,000 adult citizens, or about 36,000, who are relatively unknown outside their immediate circle of family and friends, will go unnoticed in the population at large; certainly so outside their home cities or states.
The total could go very high, perhaps to 100K (one in 1800), before the magnitude of it becomes apparent to the population at large, at least a third of whom would react with "good riddance".
It's not outside the realm of possibility. Remember that Ayers et. al. contemplated "eliminating" (killing) 25M "hard-core capitalists".