This will be tough without the subscription product. You could try putting your API calls inside child jobs, passing your parameters to the child jobs using context variables, and ticking "use an independent process to run subjob". This will potentially allow them to run in parallel. It's more of a hack than a solution I'm afraid. Parallelisation is one of the pieces of functionality that is made easier with the subscription product. You should consider how many parallel calls you wish to make at one time though. A rule of thumb would be n-1 where n is the number of cores running your job.
