const UInt_t nNumbers = 20000000U;

const UInt_t nWorkers = 4U;

auto workItem = [](UInt_t workerID) {
   // One generator, file and ntuple per worker
   TRandom3 workerRndm(workerID); // Change the seed
   TFile f(Form("myFile_mp001_%u.root", workerID), "RECREATE");
   TH1F h(Form("myHisto_%u", workerID), "The Histogram", 64, -4, 4);
   for (UInt_t i = 0; i < nNumbers; ++i) {
      h.Fill(workerRndm.Gaus());
   }
   h.Write();
   return 0;
};

ROOT::TProcessExecutor workers(nWorkers);

workers.Map(workItem, ROOT::TSeqI(nWorkers));

return 0;