Final answer:
Dehydration and polycythemia vera are two causes of excess red blood cells (polycythemia).
Step-by-step explanation:
Excess red blood cells, or an elevated RBC count, is known as polycythemia. Two causes of this disorder are dehydration and a bone marrow disease called polycythemia vera. Dehydration can lead to transient polycythemia, where the elevated RBC count is a result of inadequate water intake or excessive water losses. On the other hand, polycythemia vera is a rare disease that causes an excessive production of immature erythrocytes.