Below you'll find a bunch of ways to setup notifications for yourself based on the data you're sending through Segment.
Google Analytics custom alerts
You can use Google Analytics Custom Alerts to send yourself emails whenever a specific traffic segment drops below (or above) a threshold you set.
Analytics email summaries
In tools like Amplitude, KISSmetrics, Mixpanel you can setup email reports delivered to you on a daily basis. They are completely customizable, so you can keep an eye on as many events or other metrics you'd like.
Realtime Traffic Monitoring
Chartbeat and GoSquared both offer awesome real-time dashboards to see what's happening right now on your site. They both include the option to get notified when your traffic hits a certain threshold. For example, if your on-site visitors is less than 100 people, or more than 1,000.
Note: GoSquared also offers in-depth historical and user analysis. Chartbeat sticks to realtime anonymous traffic, but offers some sweet features for publishers.
The last option for alerting based off of Segment events is to use one of the email tools available on the Segment platform that offers event-triggered emails. Your options there are Customer.io, Vero, Autopilot, Outbound, Klaviyo, or Threads.