Does one "require" a blood transfusion to have a cesarean birth? No.
Is it sometimes necessary to have a blood transfusion following a cesarean birth? Yes, sometimes; and it is sometimes also necessary to have a blood transfusion following a vaginal birth too.
In fact, the greatest risk for haemorrhage is not actually during the birth, but in the first 24 hours following the birth. Postpartum haemorrhage (PPH) is defined as a blood loss of >500 mL immediately after the birth, or an excessive flow of lochia resulting in the saturation of a large pad in less than 1 hour. PPH is more common following a vaginal delivery, as there is a greater risk of having retained products with a vaginal birth. Typically during a cesarean, the surgeon cleans out the remnants of the placenta from the uterus before beginning to close the incisions.
In the several years that I have worked in the field, I have seen many incidents of postpartum haemorrhage, and only one of them was with a post-op cesarean patient who had no other risk factors.
Having said that, there are conditions that might predispose a person to an increased risk of bleeding prenatally, during delivery and in the postpartum period. For example, in the event that the placenta is implanted over the cervix (placenta previa) it is possible to have prenatal bleeding - this needs to be monitored closely, and the woman usually ends up in hospital on bed-rest. Also, in the event of the placenta separating from the uterus prematurely (abruptio placenta) extreme blood loss is a very real possibility, and usually an emergency cesarean is needed to (1) save the baby's life, and (2) save the mother's life. High blood pressure during pregnancy (pregnancy induced hypertension / PIH) can result in a condition called HELLP syndrome that is also life threatening and can cause massive bleeding and a condition called disseminated intravascular coagulation (DIC) which requires massive amounts of blood (I know of cases where women have had to receive over 30 units of blood and stayed in ICU for weeks) in order to save their lives.
Blood loss can also be excessive when the baby is quite large. The uterus can become so stretched that it cannot contract effectively after the delivery (uterine atony) and thus cannot successfully control the bleeding. This usually is noticed shortly after the birth, and is managed with IV fluids and oxytocin, sometimes ergotamine, and sometimes Hemabate.
I don't want to scare you with too much information, but in reality, if you are healthy there is a very low risk of needing a blood transfusion if you have a cesarean birth.