The authors of the paper have broken the body down by organs and cell types. They’ve scoured scientific literature for details on the volume and density of cells in gallbladders, intestines, bone marrows etc, then came up with estimates for the total number of each kind of cell. They estimate, for example, that we have 2 billion heart muscle cells.
Adding up all their numbers, the scientists came up with 37.2 trillion cells. Impressive!
Picture credit: https://drmichaelwayne.com/blog/ramoscience.weebly.com/