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.

https://github.com/jenkinsci/ec2-plugin/blob/master/src/main/java/hudson/plugins/ec2/EC2Cloud.java#L241

If the ami id and the tag jenkins_slave_type match, it is counted toward the cap.

Scenario:

slave description: small slave
ami: ami-12345
instance type: t1.small
cap: 5

slave description: big slave
ami: ami-12345
instance type: c3.xlarge
cap: 2

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.