There are about 100 billion nerve cells or neurons in your brain's cortex. They were all there when you were born.
The theory right now is that you do not grow new brain cells. You have x amount at birth and that's it.
Every neuron is capable of sprouting up to 20,000 dendrites if they are nourished properly and stimulated.
In addition to neurons, we each have up to 900 billion glial cells which develop myelin, the sheathing that wraps around neuron axons. Axons are the nerve pathways that send messages from neuron to neuron.
We grow new brain cells in the cerebellum, the lobe in the back of the brain that is responsible for motion, moving our bodies and muscle control.
The neurons, glial cells and new brain cells in the cerebellum all depend on what we feed our bodies with.
If the pregnant mother is undernourished and further damages her body by taking drugs, smoking, drinking alcohol, etc., the baby will not be born with the normal expected 100 billion neurons.