This game lists the world population as 8 billion...VLR said 6 billion people died due to Radical-6. So 2 billion survived? That's a lot more than I thought.
That's probably accurate. VLR spoilers:
Obviously Sigma, Akane, Junpei, Clover, Phi, Alice, and many others survived. The fact that Junpei found Quark shows that since he isn't very old, he was likely born after the reactors went off. 2 billion people surviving would make sense.