how many cells do we have in our body?
The answer is 'a whole bunch!'. Wikipedia says there are about 100 Trillion = 1014 of them. You can make a crude estimate, however, without counting all of them. Individual cells can be seen in a microscope and you can measure their size, d, which is about 10 microns. The volume of a cell is then d3 and its weight is the volume times the density, close to that of water.