People living at sea level have around 5 million RBC per cubic millimeter of their blood whereas those living at an altitude of 5400 metres have around 8 million. This is because at high altitude -

At high altitude the atmospheric pressure is low, so the partial pressure of oxygen in the air is low.
Less oxygen enters the blood with each breath, and the tissues tend to receive less oxygen (hypoxia).
The low oxygen level stimulates the kidneys to secrete the hormone erythropoietin.
Erythropoietin stimulates the bone marrow to produce more red blood cells.
With more RBCs, and therefore more haemoglobin, the blood can pick up and carry the required amount of oxygen even from the thin air.
It has nothing to do with pollution-free air, UV radiation or more nutritive food.
So at high altitude the atmospheric $O_2$ level is less, and hence more RBCs are needed to absorb the required amount of $O_2$ to survive.