There are four main types of glial cells. Schwann cells in nerves and oligodendrocytes in the brain and spinal cord, both of which form myelin insulation on axons. Plus, astrocytes and microglia. The microglia protect the brain from injury and disease, their own body guard. On average there are about 6 glial cells for every neuron, although they come in differing ratios in different parts of the brain. (Fields, R. Douglas, PhD. Pg 23. NY: Simon & Schuster, 2010)