How many cells are there in the human body?
This is a pretty tricky question, as anatomy is highly variable from person to person as well as over the course of a lifetime. Even as I sit here, typing this answer, some of the cells in my body are dying or being generated for the first time.
According to the Smithsonian Magazine, there are approximately 37.2 trillion cells in an anatomically normal, adult human body. This is a difficult number to calculate, as different kinds of tissues and cells may have different densities. If all cells had precisely the same weight and density, we might be able to take the weight of a certain number of cells (say, one thousand) and divide a person's weight by that number to find out how many thousands of cells are in that person's body. But this is not the case, so scientists have made their best guess factoring in these differences in density.
Even if our best guess tells us that 37.2 trillion cells comprising the human body, there are many more lurking inside of our digestive tracts, sinus cavities, and even on our skin! Microbes help maintain a healthy human body, so no worries about these guys.