How much blood is in the stomach?

Depends how hungry the vampire was.

(The truth is, bleeding in the digestive tract requires a visit to the doctor.  The blood should stay in the stomach wall, and not enter the stomach.)

https://medlineplus.gov/ency/article/003118.htm

http://patient.info/health/vomiting-blood-haematemesis

42 ounces.

I'm just joking. 

Happy and Angela have the correct answers and Happy has giving you a very good link.  

That would depend on how many live animals I'vr just eaten.

There should be NO blood in the stomach.