Automatic horizontal scaling can be implemented through tunable triggers, which monitor the changes in the application load and increase or decrease the number of nodes due to them.
To configure a trigger for the automatic horizontal scaling open Settings > Monitoring > Auto Horizontal Scaling section and press Add button.
In this way, you can configure the triggers for specific stacks and resources (CPU, RAM, Network, Disk) within your environment separately. Just specify the conditions of scaling and apply changes.
Please, find more details about automatic horizontal scaling in the corresponding document.