aggregator.cs

...
var jobs = db.GetJobs(..);
var aggregator = sys.ActorOf(Props.Create(() => new AggregatorActor(jobs.Count));
foreach(var job in jobs)
{
     workerRouter.Tell(new JobMessage(job.Data, aggregator);
}

...


class AggregatorActor : ReceiveActor 
{
    public AggregatorActor(int jobCount)
    {
        Receive<JobDone>(msg => {
            jobCount --;
            if (jobCount == 0) //done
            {
                //Tell someone
            }
        });
    }
}