Alerts are sent once per scan each time they are triggered. This means that if you configure an alert to trigger if SSH ports are open, for example, and then change your connected account's scan frequency to hourly (only paid plans can adjust frequency), you may receive up to one alert per hour until the SSH port risk is resolved or the alert is disabled. If multiple resources are affected, you will only receive one email listing those resources (as opposed to one email per resource per scan).