How often do I need a blood transfusion

It deoends. It depends what you need for? For example some people need it because they are bleeding from some were, if the bleeding continues they have to give blood transfusion tell the bleeding stop and your blood level get high enough to support your system. That could one time or 10 times.
It depends. People generally don't need transfusions, unless they have loss of blood or conditions, like anemia, or surgical procedures that require a great deal of blood replacement, and these are often done by autologous blood transfusion where the patient donates his/her blood ahead of time for the procedure in question.