The average human brain has about 86 billion neurons(or nerve cells) and many more neuroglia (or glial cells) which serve to support and protect the neurons (although see the end of this page for more information on glial cells). Each neuron may be connected to up to 10,000 other neurons, passing signals to each other via as many as 1,000 trillion synaptic connections, equivalent by some estimates to a computer with a 1 trillion bit per second processor. Estimates of the human brain’s memory capacity vary wildly from 1 to 1,000 terabytes (for comparison, the 19 million volumes in the US Library of Congress represents about 10 terabytes of data).
https://human-memory.net/brain-neurons-synapses/
While adding neurons is not as prevalent (as currently understood) adding neuro connections with synapses occurs through out our life via neuroplasticity.
Neuroplasticity, also known as brain plasticity, neuroelasticity, or neural plasticity, is the ability of the brain to change continuously throughout an individual's life, e.g., brain activity associated with a given function can be transferred to a different location, the proportion of grey matter can change, and synapses may strengthen or weaken over time. The aim of neuroplasticity is to optimize the neural networks during phylogenesis, ontogeny, and physiological learning, as well as after a brain injury.[1]
https://en.m.wikipedia.org/wiki/Neuroplasticity