app.dart

main() {
  genJobs(int activeWorkers, List<int> numbersOfAlreadyActiveJobs) {
    final maxJobs = 16;
    List<int> jobs = List<int>.generate(maxJobs, (i) => i + 1);
    final workerCapacity = (jobs.length / activeWorkers).floor();

    jobs.removeWhere((item) => numbersOfAlreadyActiveJobs.contains(item));

    if(jobs.length > 0) {  // если нужно что-то запускать
      for (var workerNumber = 0; workerNumber < activeWorkers; workerNumber++) {
          final startIndex = workerNumber * workerCapacity;
          final endIndex = startIndex + workerCapacity;
          final chunk = jobs.sublist(
            startIndex,
            endIndex > jobs.length || workerNumber == activeWorkers - 1
                ? jobs.length
                : endIndex,
          );
          
        }  


      }

      return null;
     
  }

  genJobs(8, [7, 8, 3, 4]);

}

If workerCapacity depends on length of jobs, you should calculate it after removing items from jobs. And remember to check whether the boundary conditions are satisfied or not.

main() {
  print(genJobs(8, [7, 8, 3, 4]));
}

Map<int, List<int>> genJobs(
  int activeWorkers,
  List<int> numbersOfAlreadyActiveJobs,
) {
  var chunks = <int, List<int>>{};
  final maxJobs = 16;
  activeWorkers = activeWorkers < 1 ? 1 : activeWorkers; // there should be at least 1 active worker
  List<int> jobs = List<int>.generate(maxJobs, (i) => i + 1);

  jobs.removeWhere((item) => numbersOfAlreadyActiveJobs.contains(item));
  if (jobs.length == 0) { // if there is no more jobs to run, return an empty map
    return chunks;
  }

  var workerCapacity = (jobs.length / activeWorkers).floor();
  workerCapacity = workerCapacity < 1 ? 1 : workerCapacity; // if there are more active workers than jobs, then each worker can handle one job in order to avoid overwork of workers

  for (var workerNumber = 0; workerNumber < activeWorkers; workerNumber++) {
    final startIndex = workerNumber * workerCapacity;
    final endIndex = startIndex + workerCapacity;
    final chunk = jobs.sublist(
      startIndex,
      endIndex > jobs.length || workerNumber == activeWorkers - 1
          ? jobs.length
          : endIndex,
    );
    chunks.addAll({workerNumber: chunk});
  }

  return chunks;
}