An alert rule can have multiple VMs as target. Also mentioned here at Azure documentation: https://docs.microsoft.com/en-us/azure/azure-monitor/platform/alerts-overview The following are key attributes of an alert rule:
Target Resource - Defines the scope and signals available for alerting. A target can be any Azure resource. Example targets:
Virtual machines.
Storage accounts.
Log Analytics workspace.
Application Insights.
For certain resources (like virtual machines), you can specify multiple resources as the target of the alert rule.
Also, as per another Azure documentation - Various alerts may use the same action group or different action groups depending on the user's requirements.