How much is implantation bleeding how much blood is it?

Usually less than nl. Menstruation happens because the part of the ovary having released an egg stops releasing progesterone. Pregnancy stops menstruation because the placenta release HCG and other hormones which stabilize the uterus like progesterone. If the ovary stops early or the placenta starts late, implantation bleeding may occur, usually less than a normal period, more like staining.