As much as your body needs to make on any given day. Your body keeps 5 liters of blood in it. If you loose one liter one day then your body will make one liter that day to bring it back up to 5 liters. If you loose too much blood too fast ,faster than your body can make it, you will bleed to death, this is why people have to have blood donations sometimes to keep them alive.
On average a normal person at 140 =150 lbs produces about 30 units a day, that is a healthy person..
1000000 pounds I think