Hematocrit levels and polycythemia

I can also think of cancer being another cause of more red blood cells than usual, although most of them would likely be dysfunctional or not in their usual shape. There's also the case of "Polycythemia vera", which again is a disease that leads to more red blood cells than usual. As you sort of pointed out, homeostasis of red blood cell production is tightly controlled. Unless under a condition of low atmospheric pressure, low oxygen occurs (including congenital heart disease in adults, COPD), there's no need for your body to expend more energy creating more red blood cells.