There are a couple of options that might work depending on how you're trying to configure the alert behavior.
--Trigger only when the event count is lower than 100:
You can look into setting up a trigger condition that evaluates how many results were returned from the search. I'm not sure what kind of alert you have, but here is some documentation that might help:
--Reduce how often the alert triggers:
You can look into throttling the alert to reduce the alert triggering frequency. Here is some documentation that might help:
Hope this helps! Let me know if you have further questions.
Try something like this
Your base search looking for that event | stats count | where count>=0 AND count<100
Set your alert condition to "where count is greater than 0"