Whilst a few of the big named ones had individuals with an ASD perpetrating them, which made it seem like an overrepresentation, I recall my mother reading a report that stated it's only a little higher than the average population of those with an ASD that went on massacres (not just school ones).
So, a "normal" amount. Meaning, not much more than 1% have an ASD; the other 90%+ don't.