Computing power

Thirty billion neurons in the average human brain
Each neuron is networked to its neighbor via multiple synapses
Creating nearly one million billion synaptic connections
These connection in turn create a very large numbers of neural circuits
The actual number is Enormous, It is the number 10 followed by one million zeros
To give some kind of scale to those numbers, the total number of atoms in the entire universe is estimated to be the number ………………. …………………. 10 followed by 80 zeros 10,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,00

So.... The number of atoms in the entire universe is smaller than the possible number of neural circuits in your own brain ( by several orders of magnitude )
By extensive cross-referencing of available human neuroanatomical data we produce a consistent set of parameters for the whole brain, the,cerebral cortex, and the cerebellar cortex. By comparing these inferred values with those predicted by the expressions, we draw the following general conclusions for the human brain, cortex, cerebellum:
(i) Interior packing is less efficient than exterior packing (in a sphere).
(ii) Fully and randomly connected topologies are extremely inefficient.
More specifically we find evidence that different topologies and physical packing strategies might be used at different scales. (iii) For the human brain at a macrostructural level, modular topologies on an exterior sphere approach the data most closely. (iv) On a mesostructural level, laminarization and columnarization are evidence of the superior efficiency of organizing the wiring as sheets. (v) Within sheets, microstructures emerge in which interior models are shown to be the most efficient. With regard to interspecies similarities and differences we conjecture (vi) that the remarkable constancy of number of neurons per underlying
mm 2 of cortex may be the result of evolution minimizing interneuron distance in grey matter, and (vii) that the topologies that best fit the human brain data should not be assumed to apply to other mammals, such as the mouse for which
we show that a random topology may be feasible for the cortex

It’s not the number of brain cells that matter so much as the possible number of connections
And why is the corpus callosum so narrow (The corpus callosum is a structure in the longitudinal fissure that connects the left and right cerebral hemispheres)