Mulit-user alerting alerts multiple users at once when an incident is triggered. Multi-user alerting can be set-up under an escalation policy to alert up to 10 users or schedules at the same time.
Why is multi-user alerting limited to 10 users?
PagerDuty helps you alert and get the right people onboard to be notified of a triggered incident. Alerting more than 10 users runs the risk of creating confusion about who owns the incident.