There are loads of things humans are still largely in the dark about: the far off reaches of space, the deep recesses of the ocean, how they get the caramel in a Caramilk bar. Yet there are some things one doesn't expect to see on the list of enduring scientific mysteries—chief among them any seemingly basic thing to do with the human body.
Yet as Peter Andrey Smith writes for the Boston Globe, despite what you may have heard, we have only a vague idea of how many cells are in the human body. At best, we're in the range of two orders of magnitude. What that means in numbers: “[T]he human body has an estimated 37 trillion cells—with a considerable range that goes from 5 to 724 trillion,” Smith writes.
Some people are bigger and some people are smaller, and surely that diversity accounts for some variability. But no one person is 100 times bigger than another. The huge range on how many cells are in the body is just old-fashioned uncertainty: we just don't really know.