We need two variables for this problem
a) The fraction of human blood in one body
b) And of course, the world population
Since, blood has to carry all the oxygen and other nutrients, our body must be more than 1% blood but most certainly less than 10% blood. If we take the average, then we each have about 5% of 100 liters of blood.
Or, we each have 5 liters of blood.
The world population is about 7.8 * 10^9 billion. Thus, the total volume of blood is 5 * 7.8 * 10^9 liters.
Or, about 4 * 10^10 liters!