erythrocytosis

noun

eryth·​ro·​cy·​to·​sis i-ˌrith-rə-ˌsī-ˈtō-səs 

plural erythrocytoses -ˈtō-ˌsēz 

: an increase in the number of circulating red blood cells especially resulting from a known stimulus (as hypoxia) compare polycythemia