The setting is a society in which aging has been cured, individuals have indefinite lifespans, and population control is used to limit the population of the United States to forty million. This is maintained through a combination of infanticide and government-assisted suicide - in short, in order for someone to be born, someone must first volunteer to die. As a result, births are few and far between, and deaths occur primarily by accident.
The scene is a waiting room at the Chicago Lying-In Hospital, where Edward K. Wehling, Jr. is faced with the situation that his wife is about to give birth to triplets, but he has found only one person - his maternal grandfather - who will volunteer to die. A painter on a stepladder is redecorating the room with a mural depicting famous doctors and nurses - in particular, Dr. Benjamin Hitz, the hospital's Chief Obstetrician. Leora Duncan, from the Service Division of the Federal Bureau of Termination, arrives to pose for the mural. The mural is a picture of a garden that's well taken care of. It is a metaphor for the United States at that time. Later, Dr. Hitz enters the scene, conversing with everyone but the painter of the mural.
It becomes apparent to all that Wehling is in a state of despair, wanting not to send his grandfather and two of his children to death. Dr. Hitz questions Wehling's belief in the system, and tries to make Wehling feel better by explaining how the surviving child will "live on a happy, roomy, clean, rich planet." Suddenly, Wehling draws a revolver and kills Dr. Hitz, Leora Duncan, and himself - "making room for all three children."
The painter, who is about two hundred years old, is left to reflect on the scene, and thinks about life, war, plague, and starvation. Descending the stepladder, he initially takes the revolver, intending to kill himself, but he can't do it. The last line is from the receptionist at the Bureau:
â"Thank you, sir," said the hostess. "Your city thanks you; your country thanks you; your planet thanks you. But the deepest thanks of all is from all of the future generations."â

Question: Population control is used to limit the United States to how many people?
Answer: Forty Million.
Alternative answer: 40 million.