You need to proceed as follows to set the parallel iterations:
Simply select the Iterate link of your subjob to display the related Basic settings view of the Components tab.
Select the Enable parallel execution check box and set the number of executions to be carried out in parallel.
When executing your Job, the number of parallel iterations will be distributed onto the available processors.
Select the Statistics check box of the Run view to show the real time parallel executions on the design workspace.