I wouldn't actually be "hundreds of thousands" as you say. Go back to school and learn your VEI scale.
it most likely will not blow but it does. Say good bye. Yellowstone is measured 2,5090 times more powerful than St. Helens and would most likey place molten lava over 20,000+ square miles of land. St Helens is not even in the same category as Yellowstone, Toba, Taupo, Black Tail etc. Yellowstone is a VEI 8 (mega-colossal) where Helens is a VEI 5 (large). If you surve the initial firebombs, earthquakes and lava, then you will have about 200 300 feet of ash to look forward to.
It would more likely knock out all human life in the USA and burn/melt most of the populated areas of Canada and Mexico. Don't forget it will cause a huge drop in temperature which will cause a mini ice age Doesn't really matter where you are in the world as everyone will most likely starve to death in a couple years anyways. I think it was Mt Toba that went off last time... which dropped the human population down to 10,000 and kicked off a mini ice age 75,000 years ago.