Hi everyone Joseph here:
Today I wanted to consider how many people a benevolent vampire would have to draw blood from, assuming that the vampire requires 5 liters of human blood (the approximate amount of blood in a healthy adult), and the vampire didn’t want to permanently injure anybody.
According to John Hopkins,  approximately 1% of a person’s weight can be drawn every three weeks. For me, I’m approximate 130 pounds, which translates into approximately 60 kilograms. 1% of 60 kg would be .6L, which is surprisingly not too different from the water bottle in the picture above.
However, remember that although most components of our blood can replenish themselves and the lost blood volume in around 24 hours (plasma, white blood cells),  red blood cells can take up to four to six weeks to regenerate. For this reason, you must wait eight weeks between whole blood donations with Red Cross.
In the clinic, when donating blood, to be on the safe side, the normal volume taken is approximate 450 mL, under the threshold recommended by John Hopkins for maximal blood draw. This means, for the benevolent vampire, it would require approximately 11 volunteers that could only donate every eight weeks.
Ultimately, I think it would be quite difficult to maintain that many willing volunteers, which may contribute to vampires’ voraciousness and necessity for unwilling victims.
Your friendly vampire,