In response to increasing violent attacks, the Stop the Bleed campaign recommends that everyone have access to both personal and public bleeding-control kits. There are currently no guidelines about how many bleeding victims public sites should be equipped to treat during a mass casualty incident. We conducted a retrospective review of intentional mass casualty incidents, including shootings, stabbings, vehicle attacks, and bombings, to determine the typical number of people who might benefit from immediate hemorrhage control by a bystander before professional medical help arrives. On the basis of our analysis, we recommend that planners at public venues consider equipping their sites with supplies to treat a minimum of 20 bleeding victims during an intentional mass casualty incident.