You can see which Interface on the sensor generated the Alert in the Real Time Threat Analyzer.
Launch the RTTA and click on the Alerts tab, select the Group by option and from the Dropdown select Interface.
This will show all of the Interfaces on your sensors, sort it by device.
You can also add the interface detail to the detail view under Preferences > Alert View.
The Real Time Threat Analyzer only shows High and Medium severity Alerts so to see the actual volume you need to use the Historical Threat Analyzer.
You can also query the iv_alert table of the database directly to get a count of alerts for a device / interface.