You can place the upward limit to the number of EC2 instances that Jenkins may launch. This is useful for avoiding surprises in the billing statement.

For example, if this field is 3, Jenkins will only launch a new instance as long as total number of instances you run on EC2 (regardless of whether that is for Jenkins or for other purposes) doesn't exceed this number. In this way, even in the worst case of Jenkins starting instances and forgetting about them, you have an upper bound in the number of instances that are concurrently executed.

Leave this field empty to remove a cap (note that EC2 appears to have its own instance cap at 20.)