Depends on what time you go. Early EU daytime, is quite few ppl. usually just a party or 2 at a Mark.
Also it's not possible to camp them, since they can spawn anywhere. You can have a party spread out, and cover more ground, though.
The only thing they need to do, is scale the HP of mobs, as with fates; So they get more HP, the more ppl are doing it. That way these marks will take maybe a min to kill, instead of 3 seconds.