I lost about 2L--maybe a little more-- according to my doctor. I just know it was enough for me to pass out. I retained all of the placenta and they took too long in trying to get it out, so after I passed out they took me to surgery to remove it. I had to get 2 blood transfusions and was put on iron supplements after the delivery. (Vaginal delivery)
They told me that if they'd waited any longer, or if I had opted for a home birth, I probably would have died. Not typical, but they said now that it happened to me once, chances are it will happen to me again in future pregnancies.