[Kuruharan]

So in Middle-earth math, one Noldo > thousands of Dwarves?

I pretty much agree with what everyone else has said.

Why not? Because a large number of the Erebor survivors had fled to the Iron Hills after Smaug's arrival. Why wouldn't they have flocked to him in the Ered Luin, if not because they didn't consider the possibility of a real dwarf-kingdom existing there again to be an option? Instead, they stayed close to the Lonely Mountain, the real kingdom of Thorin.

Maybe they liked Dain better.

I also have the impression that many of the Longbeards were still in places in the Grey Mountains and other spots. Note what Bard and the Elvenking feared in the lead up to the Battle of Five Armies. They were afraid that greater and greater numbers of dwarves would show up. The only places close enough for this to matter to the military situation on the ground at that moment were the Grey Mountains (and possibly in the Mountains of Rhun). Why Thorin couldn't have gathered all of these dwarves that were seemingly in the area together to attempt to retake the mountain I don't know...unless perhaps the dwarves just didn't fancy him that much and liked Dain better.