Red blood cells are the most numerous blood cell, about 5,000,000 per microliter. Red blood cells make up about 40% of our total blood volume, a measure called the hematocrit. White blood cells are the largest of the blood cells but also the fewest. There are only 5,000 to 10,000 white blood cells per microliter. Platelets are only about 20% of the diameter of red blood cells. The normal platelet count is 150,000- 350,000 per microliter of blood, but since platelets are so small, they make up just a tiny fraction of the blood volume.