Instance counts towards the cap feature are incorrectly counted when you use the same AMI for multiple jenkins slave types it with differing ec2 instance types. The count is a sum of all the instances provisioned by the plugin rather than the number provisioned per discrete slave configuration.
If the ami id and the tag jenkins_slave_type match, it is counted toward the cap.
slave description: small slave
instance type: t1.small
slave description: big slave
instance type: c3.xlarge
If there are five "small slaves" running it won't provision any "big slaves", reports the following:
AMI Instance cap of 2 reached for ami ami-12345, not provisioning.