The Control Panel's Webcast tab is used to monitor the Broadcast (online listen only) connections to your event. You can access the Webcast tab by clicking Scheduled Events and then the Control Panel link in the Action column for the event.
During the live event the Webcast tab will show you a count of how many attendees are currently connected to the listen only connection.
After the call has ended you can use the Webcast tab to request the 'peak webcast report' for the event. This can be done anytime after the call has ended, even months later if you need.