Mojo Stratus uses Amazon Autoscaling to dynamically add resources (EC2 instances) to your store as traffic increases, or reduce them as traffic decreases. On your side, this is all hidden from view and occurs automatically. No settings or action is needed on your part to handle more traffic. Your store will scale as needed.
If you are interested in the nitty-gritty of autoscaling, check the first link in this article or the general FAQ from Amazon about autoscaling.