I dont have any stats on hand but your midwife would have synthetic oxytocin in case. I have heard from many sources (and yes this is squeamish!) that licking your placenta can stop / prevent PPH too! Google it, I am sure you will find something on it!

Your risks of PPH greatly increase with that many and subsequent c/s and also inductions but I would assume it wouldn't be as much for a normal birth. Perhaps speak to a naturopath too, there might be some things you can do now and at the birth.