From the Wiki "The hobos in the central area have roughly 400 HP. If you deal 505+ damage of a single type to them (it doesn't have to be in one hit) you will "overkill" them, leaving behind hobo parts. There are six hobo parts, one for each element, plus physical damage. Having a full set of six hobo parts allows Richard to make a scarehobo, killing ~8 hobos. Overkilling hobos effectively allows you to kill slightly more than 2 hobos at a time. However, if you overkill hobos you will get no normal drops, including nickels. So, scarehobos are a choice between speed and loot."
I think they equal to 8 hobos.
so 6 adventure to get all the parts, plus over killing kills about 2 at a time.
So, 1 Scarehobo (6*2)+8= 20 hobo's