https://medium.freecodecamp.org/how-to-schedule-ad-hoc-tasks-with-dynamodb-ttl-and-lambda-421fa5778993

CloudWatch Events let you easily create cron jobs with Lambda. However, it’s not designed for running lots of ad-hoc tasks, each to be executed once, at a specific time. The default limit on CloudWatch Events is a lowly 100 rules per region per account. It’s a soft limit, so it’s possible to request a limit increase. But the low initial limit suggests it’s not designed for use cases where you need to schedule millions of ad-hoc tasks.

CloudWatch Events is designed for executing recurring tasks.