There are limits on the number of Spot Instances that can be launched and requested per AWS account, depending on the region.
The Spot Instance limit is managed based on the number of vCPUs (virtual CPUs) used by running Spot Instances or pending Spot Instance requests that are waiting to be fulfilled. If you terminate a Spot Instance but do not cancel the Spot Instance request, it will continue to count toward the Spot Instance vCPU limit until Amazon EC2 detects the termination and closes the request.