So I'm writing a post apocalyptic story. Basically, a war happens and after a solar storm there's very few human survivors on Earth. This world has "magic" crystals, rocks that emanate energy. The majority of the world population would sadly die. But even if the war was affecting the whole world, it would be impossible for that to happen. I'm thinking about a lot less than a billion humans surviving, about a million, but I don't know if that would be enough for the scattered human survivors to restart humanity. What things would need to happen? I'm thinking a few big nuclear bombs that were launched in the war, and a solar storm that would "turn on" the magic crystals, making them release a lot of energy that can do a lot of damage and mutate DNA, but the mutations are more positive than the ones that radiation does.
My question is: what else would need to happen for the human population to shrink that much? And would about a million humans be able to remake humanity in about ten thousand years?
I'm sorry if my English is bad, I'm not a native English speaker. And if my question is too broad please tell me in the comments.