How many neurons in a adult's brain?

A good estimate of the number of neurons in an average adult brain is about 85 000, 000, 000 (89 billion - but ‘a billion’ is too big for us to imagine to begin with). The Search for True Numbers of Neurons and Glial Cells in the Human Brain: A Review of 150 Years of Cell Counting

Now, each neuron might have 100, 000 dendritic connections, however these densely-connected neurons are invariably found in the cerebral cortex. Hence, the number of synapses in the vortex in general is estimated to be 1.8-3.2 x 10¹⁴.

Hence, the Scale of the Human Brain presents peculiar problems, especially (ironically) in the conceptualisation of that brain via the use of it.

Next time you see a diagram like this, try counting to a few hundred trillion.

Synapses & Circuits