There are trillions of cells EVERYWHERE including blood cells, brain cells, and even skin cells !! I believe that multicellular organisms that are large need to have trillions of cells to be able to fill their whole body. For example , humans, Humans have one of the most complex cells, we humans are very big so thousands of cells would not be enough to contain our entire body. Even the word ' multicellular' meaning multiple cells gives people a hint that we are multicellular . In conclusion , trillions of cells are needed for multicellular organisms , without them we wouldn't be able to function properly.