Why does a person living at a high altitude need large amounts of red blood cells

The effects of high altitude on humans are considerable. The oxygen saturation of hemoglobin determines the content of oxygen in blood. After the human body reaches around 2,100 meters (6,900 ft) above sea level, the saturation of oxyhemoglobin begins to decrease There’s oxygen in the air at high altitude. An increase in red blood cells in their bodies allows people to increase their body oxygen levels, especially when they perform strenuous exercise at high altitudes. Many athletes will train at high altitudes in preparation for competitions at lower altitudes. However, the human body has both short-term and long-term adaptations to altitude that allow it to partially compensate for the lack of oxygen. There is a limit to the level of adaptation; mountaineers refer to the altitudes above 8,000 meters (26,000 ft) as the death zone, where it is generally believed that no human body can acclimatize

(I'm so sorry if I'm wrong)

Hope this Helps!