This is based on the idea of a society where vampires are known and subsist off of 'willingly' given human blood. An average adult human needs approximately 2000 calories a day to maintain their weight, and so would a vampire. There are 900 calories per liter of blood, and about 5 liters of blood in the human body. A vampire would essentially need 2 - 2.5 liters a blood a day...or approximately 4 - 5 pints of blood. However, if donating platelets is healthier/more efficient for humans than donating whole blood then I may change the vampire's nutritional needs to be focused on needing a certain number of platelets rather than calories per liter of blood.
I know that these are the standard donation practices according the redcrossblood.org, but I can't find anything specific about how much blood you can lose every day and still be healthy.
You must wait at least eight weeks (56 days) between donations of whole blood and 16 weeks (112 days) between Power Red donations. Platelet apheresis donors may give every 7 days up to 24 times per year.
So, how much whole blood/platelets would a single human be able to give every day, consistently, without dying/becoming unhealthy?
Since the time is less for those donating platelets, I'm assuming humans can donate platelets more often than whole blood while still remaining relatively healthy. If so how much?