The production of RBCs are carried out by the stem cells in the red bone marrow. The production of RBC is not determined by the number of RBCs present in the blood stream; on the other hand, kidneys detect even the slightest of variations of the oxygen in blood. If the oxygen level in blood is lower the kidneys produce the hormone called erythropoietin which in turn stimulate the red bone marrow to produce more RBC to meet the more oxygen demand.
But too much of RBCs in the blood too is not good, it leads to high viscosity of blood which in turn impedes the free flow of blood and in extreme cases can lead to polycythemia which can cause stroke.
It is a known fact that higher altitude can trigger excess RBC production as the air is rarefied and less oxygen supply to the tissues via lungs.It is advisable to do practice of pranayama that can increase the oxygen carrying capacity of the body without increasing the RBC production and be functionally fit.