So the human body has what, 7 or 8ish pints of blood? How much is generally in the head at any one time?
Need answer fast
(Not really)
About 25% of the body’s blood flow goes to the brain. That’s a good guideline.
So, hypothetically of course, if I were to lop the head off of the jackass in the car in front of me for continued jackassery, there would be about 2 pints of blood to deal with before I could carry it around in a sack like medusa?
Please don’t be texting in the car whilst driving. 
Flow and total volume aren’t directly related like that. If I put a splitter on a hose and divert 75% of the flow into a 5 gallon bucket and 25% of the flow into a pint glass, then collect all the overflow from both to recirculate via the hose again, at steady state the pint glass will get 25% of the flow but only hold 2% of the volume (1/41 pints).
Cerebral blood volume, the amount of blood in a given amount of brain tissue, is very roughly 4 ml/100 g. So weigh a wet brain and back-calculate from there to get the blood volume in the brain parenchyma: surprisingly low at about 56 ml for a 1400g brain. That wouldn’t count the blood in the sinuses or vessels on the brain surface, or all the vessels of the scalp, face, neck, etc.
I’d be more worried about the spray from the part of the corpse that still had a beating heart in it.
Interestingly, while a lot of blood goes through the head, it doesn’t really enter the brain due to something called the Blood-Brain Barrier. This adaptation is one fo the reasons it’s fairly uncommon to get serious brain illnesses - most brain problems come from major internal problems, severe trauma, or maybe cancer. The BBB prevents anything in your blood from getting into the brain unless the body wants it.
This is why, for instance, most viruses and bacteria simply can’t affect the brain directly.
My dad fell and cracked his head open last night, and I was standing in the ER with him and thinking “Wow! How timely was that thread?!”
Luckily we didn’t get to see how much blood was in his skull, but the amount of blood in one’s scalp alone is astonishing.
Actually…the doc who stitched him up (10 stitches) said “the scalp is very vascular” which I assume means you’re not carrying massive amounts of blood in your scalp but there’s a lot traveling through. It was a very bloody scene. At least a pint, I’d say…