AWS Auto Scaling is the AWS service that allows users to optimize the number of EC2 instances based on the usage pattern, as it automatically adjusts the capacity to maintain steady and predictable performance at the lowest possible cost. Spot Instances are a way to reduce the cost ofEC2 instances by bidding on unused EC2 capacity, but they are not suitable for applications that require steady and reliable performance. Reserved Instances are a way to reduce the cost of EC2 instances by committing to a certain amount of usage for a period of time, but they are not flexible to adjust to the usage pattern. AWS CloudFormation is a way to automate the creation and management of AWS resources, but it does not optimize the number of EC2 instances based on the usage pattern. These concepts are explained in the AWS Cloud Practitioner Essentials course3.