How Many Neurons Does the Average Human Have?
The 100 Billion Neuron Myth
Humans have about 86 billion neurons, not 100 billion.
It turns out the long-cited “100 billion and 10 more glial cells” line is junk science and lacks citation to any peer-reviewed study. Only one real study has been done on neuron counts, and that study showed the average male has about 86 billion neurons (+/- 8.1 billion) and 84.6 (+/- 9.8 billion) non-neuronal cells (glial cells). No study has ever shown 100 billion, although a study was done that myth-bused the 100 billion neuron claim and backed up the 86 billion neuron claim. With this in mind, the best we can do is give a rough estimate when teaching or writing. The exact number of neurons in the average human won’t be known without further studies to confirm the one singular study done.  
- The 2009 study that explains how many neurons the average human has: Equal numbers of neuronal and nonneuronal cells make the human brain an isometrically scaled-up primate brain.
- The paper that explains that the 100 billion neurons myth was never backed by citation: The Human Brain in Numbers: A Linearly Scaled-up Primate Brain.
86 Billion, Not 100 Billion Neurons (According to Peer-Reviewed Studies)
Here is an excerpt from the 2009 study mentioned above that shows the average human male has about 86.1 (plus or minus 8.1) billion neurons (not 100 billion):
Here is an excerpt from the study that pointed out that the 100 billion neurons myth had persisted despite no one ever actually knowing the count before the 2009 findings:
Revising the numbers. How researchers got to a figure of 86 billion neurons, instead of the 100 billions previously estimated.
FACT: For comparison a jellyfish has about 5.6 thousand neurons, a cockroach has about 1 million, a house mouse has about 71 million, a squirrel monkey about 3.24 billion, and an African elephant 267 billion. 
NOTE: Interestingly, the study that found the count of neurons also makes a comment on evolution: “This is the same ratio found in primates. These findings challenge the conventional view that humans stand out from other primates in their brain composition and indicate that, about numbers of neuronal and nonneuronal cells, the human brain is an isometrically scaled-up primate brain.”
Do We Grow New Neurons? Is That Why the Old Count Was Off?
Some parts of the brain contain neural stem cells. These stem cells continue to grow new neurons. The more you use the parts of your brain that contain these cells, the more new cells are produced. Despite this number of neurons a person has doesn’t change much from birth, although you can grow more neurons in certain parts of the brain, so it is unlikely this is the source of the number being off. 
TED Talks explains how you can grow new brain cells (neurons).
What are Neurons?
Neurons (or nerve cells) are “electrically excitable cells” that process and transmit information through electrical and chemical signals over synapses. Neurons are one of the main core components of the brain; they can also be found in the spinal cord, central nervous system, and throughout the body.
Each neuron connects to other neurons and cells via up to as many as 5,000 – 10,000 synapses creating a “neural network” of sensory information. 
A video explaining neurons in two minutes or less.
FACT: Synapses don’t just connect neurons to each other; they connect them to 1,000-10,000 other neurons, muscle cells, glands, etc.
What are Glial Cells?
Glial cells are the supporter cells of neurons.
How Do We Store Information in Neurons?
When we perceive sensory input we store it as memories in our neurons (blips of sensory information, connected to through synapses, that our brain makes sense of through memory recall).
Each neuron is designed (via it’s DNA) to encode a different type of sensory information, so each of the many neuron types will be primed to store a particular type of sensory information.
When two neurons receive information at the same time, they connect to each other via synapses forming or strengthening “synaptic connections”. These synaptic connections between neurons form complex neural networks. These systems act as pathways associating bits of sensory information together to recall as needed.
How Do We Retrieve Information From Neurons?
When we recall a memory, we are grabbing sensory information from many different synaptically connected neurons. The brain pieces this information together to repaint a picture one sensory memory at a time. In other words, memories aren’t just one thing, but rather it’s many neurons and their connections that “store” a single memory.
It’s all in the Nervous System
Neurons relate to many different types of sensory memories, and there are many different types of neurons throughout our bodies’ nervous system. All of our memories, creativity, thought, ideas, abilities, and most of who we are “stems” from information stored in our neurons and the X trillion synaptic connections between them.