What is the relationship between the number of erythrocytes in human blood and the height above sea level?

What is the relationship between the number of erythrocytes in human blood and the height above sea level? What does a person experience when moving from low-lying areas to high-mountainous ones? In what cases does the number of red blood cells change in the blood?

1) The number of erythrocytes in the blood increases significantly at high altitudes above sea level, since at high altitudes the air is rarefied, there is a lack of oxygen in it
2) when moving from low-lying areas to high-mountainous areas in the first days, a person experiences weakness, dizziness, decreased performance
3) the number of red blood cells in the blood changes when climbing mountains, muscular work, bleeding; anemia, lack of vitamin B12, iron and with age