Don't know about 42-500 by I can give you some figures for -300, hope this might help.
For 800m RWY, 0ft elevation and 33°C, MTOW is 13 500kg and MLW about 14t (for ALD/RLD=0.7). ATRs I fly have DOW 11200 kg which leaves 2300 kg for fuel and payload. 170 Nm trip with alternates near to destination requires 1200 kg take-off fuel and 340 Nm about 1600. Typical landing fuel at destination is 700-800 kg. Therefore 42-300 would be able to take out 13 pax on a 170 Nm trip, 8 on 340 Nm one and bring in about 23 pax.
As -500 series has 30% more power I suspect it would be able to pick up some more passengers, but whether it's one, two or five - we'll have to wait for someone current on -500 to answer.