https://docs.aws.amazon.com/appstream2/latest/developerguide/autoscaling.htm
Fleet Auto Scaling for Amazon AppStream 2.0 allows users to adjust the size of their AppStream 2.0 Mas-On or On-Demand fleets to match the supply of instances to user demand. The size of the fleet determines the number of concurrent users who can stream. Fleet Auto Scaling uses Application Auto Scaling, and requires permission to access Amazon CloudWatch alarms and AppStream 2.0 fleets. Users can manage fleet scaling using the AppStream 2.0 console or the AWS CLI. To use Fleet Auto Scaling, users must understand concepts such as minimum capacity, maximum capacity, desired capacity, scaling policy action, scaling policy condition, and scaling policy metric. To set a fleet scaling policy, users can edit existing policies or add new policies in the AppStream 2.0 console. The Fleet Usage tab can be used to monitor the effects of scaling policy changes.