amount of blood per period: https://www.menstrual-matters.com/bloodloss/ (35 - 40ml, i took the higher estimate, sources for the values at the bottom)
amount of periods in total: https://pmc.ncbi.nlm.nih.gov/articles/PMC2244694/ (451.3, rounded down to 450)
40ml * 450 = 18000ml = 18l
I can't find the original numbers for average blood content in a human but the general consensus seems to be that it's between 4.5 and 5.5. It doesn't really matter though since, even if we say that you'd sacrifice 4 people with only 4.5l of blood, you'd still reach your desired 18l of sacrificial blood.
True but it's not only blood. Source 1 says it's 70-80ml of fluids, 35-40ml of which is blood.
Regardless, as someone who has an almost obsessive fascination with our beautiful red fluid, the idea of someone being squeamish about blood is wild to me