Probably even a few hundred. I'm sure there's probably some information somewhere about how few people you'd need to survive for humanity to flourish. Obviously the smaller the pool the greater the chance of genetic anomolies being passed along.
Even if 700,000 survived I guess the difficulty would be that they won't all be in one place, in fact it's likely you'd never be able to draw them all together, either because of the problems in trying to contact everyone, or the logistics of getting together. How would survivors in Brazil link up with those in the UK for example.
And actually you may not want them too, scattered settlements might be better than one big city of humans. It's too easy for one settlement to be destroyed I guess.
But yeah it'd have to be one hell of a virulent inefection to kill every single human. And forget Madagascar , Greenland's the place to be. Those gits have thwarted my plans on Plague inc (great game) more times that any other country!