Why is it that large important complex systems often have about 10-100 billion (or 1E10 to 1E11 give or take an order of magnitude) objects? This seems to apply to the number of stars in the Milky Way (3E11), the number of galaxies in the observable universe (2E11), the number of cells in the first mammal (1E10), the number of neurons in a human brain (1E11), the number of nuclide pairs in human DNA (3E9), the number of proteins in a typical eukaryotic cell (1E10), and the current population of earth (7E9). Yes, many of these are not well determined yet and various estimates exist.
In a complex system there are hierarchies, e.g., organs, tissue types, and cells in humans. Variation is needed such that certain objects in the system can specialize and support the hierarchy.