[marktwain]

Here are 27 cases:

http://gunwatch.blogspot.com/2015/05/mass-killings-stopped-by-armed-citizens.html

[richardb72]

All the cases here from the last decade were from the crimeresearch.org website.
https://crimeresearch.org/2016/09/uber-driver-in-chicago-stops-mass-public-shooting/