Short answer to the question suggested by the title: The neuron density (TND) is reduced relative to non-autistic subjects. But of course the full story is much more complex and nuanced than that one measure.
Even TND appears to be very regionally specific and may have important differences across development in autism. The most exciting thing are differential findings of density of dendrites vs neuron cell bodies. We haven't looked at that so much in autism and when we take that into account, suddenly a bunch of new regions pop up.