How many red blood cells are there in an average 70-kg person?

There are ∼2.6 × 1013, or 26 trillion. The total blood volume is ∼5.5 L, and ~40% of that is red blood cells;i.e., there are ~2.2 L of red cells. Since each cell has a volume of ∼86 × 10−15 L , the result follows from dividing 2.2 L by this number.