If spleen is graveyard of RBC then till now rbcs would have been accumulated over the spleen hence they must move out the body, so how they manage to do so?

The spleen is not the only organ which removes "old" erythrocytes, this happens as well in the liver and the lymph nodes. The whole process is termed eryptosis, the Apoptosis of Erythrocytes. During the aging of erythrocytes sialic acid on their outer membrane surface is removed. This leads to the recognition by macrophages and phagocytosis of this cells by macrophages which are located in liver and spleen.Polycythemia is the state when you have an elevated number of red blood cells, characterized by an increased Hematokrit level. This is caused by the over-production of red blood cells, which can be caused by an overproduction of hematopietic cells in the bone marrow (so called myeloproliferative syndrome, which are basically cancers), the exposure to permanently low oxygen-levels (this is what athletes exploit when the do training at high altitudes) or malignancies (usually lymphoma). It is also possible due to the mis-use of Erythropoietin (EPO) which induces the production of red blood cells. Another possibility for having too much red blood cells is the over-transfusion during a blood transfusion.