The moment you need more instances, you will have to contact the Runinstances. Right from your call the Amazon EC2 will typically set your new instances in the shortest time possible. This speed is due to the fact that the services are control by the web services or the APIs. Your capacity can automatically scale itself up and down depending on your requirements.
The Amazon EC2 offers an elastic compute environment. The EC2 allows you to decrease or increase capacity within a short time range not exceeding hours. Commissioning hundreds or thousands of server instances simultaneously is possible since this is a web controlled service. The applications you want will automatically scale themselves up and down depending on what it requires or needs.