According to a National Geographic article, the best available estimate of the number of cells contained in the human body is 37.2 trillion. The researchers who conducted the study added up the estimates for the total number of each type of cell to arrive at the total estimate.
The National Geographic article states that actually counting the number of cells in the human body is nearly impossible. This is true because it is difficult to identify particular sorts of cells, such as tangled neurons. It is also impossible because, at a rate of 10 cells a second, tens of thousands of years are required to count all the cells. As a result, estimating the number of cells is the only feasible approach.