Question

How did scientists count the number of cells in an average human body?

Answer 1

You can use methods like volume estimate or weight estimate that the scientists actually use.
volume estimate is by estimating the volume of an average adult and dividing it by the average volume of a single cell.
And weight estimate is the same thing with the weights.
Calculating the number of cells in the human body is tricky. Part of the problem is that using different metrics gets you very different outcomes. Guessing based on volume gets you an estimate of 15 trillion cells; estimate by weight and you end up with 70 trillion.
So if you pick volume or weight, you get drastically different numbers. Making matters worse, our bodies are not packed with cells in a uniform way, like a jar full of jellybeans. Cells come in different sizes, and they grow in different densities. Look at a beaker of blood, for example, and you’ll find that the red blood cells are packed tight. If you used their density to estimate the cells in a human body, you’d come to a staggering 724 trillion cells. Skin cells, on the other hand, are so sparse that they’d give you a paltry estimate of 35 billion cells.

Its actually more of averaging the results obtained by using different methods.