How many neurons does the human brain have?

The adult human brain has about 86 billion neurons and a similar number of non-neuronal cells, measured by counting cell nuclei in dissolved brain tissue. The cerebellum holds about 80% of the neurons but only 10% of the mass. Each cortical neuron connects to thousands of others, giving the brain on the order of 100 trillion synapses.

Source: Azevedo et al. (2009), Journal of Comparative Neurology