400cc of blood is 400 milliliters, and usually two units of red blood cells is about 400 milliliters. How many units of red blood cells need to be transfused for clinical anemia depends on the degree of the patient’s hemoglobin and the patient’s condition. Generally speaking, patients with mild anemia do not need transfusion of red blood cells without obvious clinical symptoms, while patients with moderate or even severe anemia often have a series of clinical symptoms such as dizziness, fatigue, chest tightness, shortness of breath after activity, etc., and need to be transfused with red blood cells to support the treatment and improve the symptoms. Patients with mild anemia can be transfused with two units of red blood cells, while patients with severe anemia can be transfused with four or even six units of red blood cells. Patients with anemia are advised to consult the hematology department to clarify the cause of anemia for targeted treatment.