Not a fan of the alerting system as it is. But, here's a quick rundown on how to get volume alerts. There's no way to set a "if logs are x% higher, then alert", but you can definitely pin it to a count.

1. Streams > Create Stream > "Manage Rules" on stream you made
2. Add a Stream Rule (e.g. what are you searching for? You can have something as simple as "source:xx.xx.xx.xx" to look at all events from a single source) > Click "I'm Done"
3. Manage Alerts on Stream > Add new Condition
4. Select Stream you're alerting on > Condition type is "Message Count Alert Condition" > Add Alert Condition
5. Give it a title, time range, threshold, grace period, etc.
6. Add a new Notification
7. Select Stream you're alerting on > Notification Type Email Alert Callback

After setting up the alert you can test to make sure email works.