I seem to remember that Capeci once quoted a book ( East side West side, I believe it was called) which proved that the number of hits for Murder Inc. were hugely inflated. Still, for todays standards, as Sonnyblackstein wrote, the numbers would be huge. But it's hard to give a good number for those old guys.
Would Nicoletti and/or Alderisio have a shot at the title? And how would it compare to, for instance, Bugsy Siegel who was a stone killer in his younger days? (anyone have any figures about him? I just know some stories which might as well by mythical.)