EC2 restricts the number of instances that can be running at a time.  Each region manges this limit separately, and the limit can be increased either from the limits page or by contacting support.

EC2 limits both the total number of instances and the number of each type of instance. The EC2 API does not expose the per-instance-type limit, but Blue Matador automatically monitors the limit on total number of instances per region and warns when you are approaching the limit.

When you are at one of the EC2 limits, you will be unable to launch more instances. To get around this, request a limit increase, or stop/terminate some unused instances in the region to make room for more.