Stereological counting applied to the brain of a long-finned pilot whale returned approximately 37.2 billion neocortical neurons and around 127 billion glial cells. The comparable human figure is roughly 16 billion neocortical neurons. The pilot whale total is the highest yet measured for any mammal.
The cetacean cortex is built differently. It is broad and heavily folded but thin, with a distinctive layer structure and, notably, no layer four, the input layer that in most mammals receives signals from the thalamus. Neuron density is lower than in primates, so a much larger surface area yields the higher absolute count. Body size is part of the story too: a six-metre animal has more body to represent.
Neuron count is a weak proxy for cognitive ability, and the pilot whale result rests on very few brains. What it does establish is that the primate pattern of packing neurons densely is not the only route to a large cortical population.