Oligodendrocytes form myelin sheath in CNS. The cell -to-cell ratio (oligodendrocyte: neuron) for this myelination is:-

Oligodendrocytes and Schwann cells are involved in myelin formation around axons in the CNS and peripheral nervous system, respectively. Oligodendrocytes are small with relatively few processes. Those in the white matter provide myelin, and those in the gray matter suppo neurons. One Oligodendrocytes myelinates 20 axons in CNS. This is the reason why CNS neurons doesn't repair.