CONCEPT: STARTING OVER

One theory that has always interested me was the thought of how the human race would start over in the aftermath of an apocalypse. What would be the ideal number of humans to serve as a starting population? Who would survive the catastrophe? How would civilization start over? In several scientific articles concerning the issue, I noticed that a number of experts stated that a starting population would need to be tens of thousands in order to guarantee genetic diversity. When the world ends and this starting population survives, it is an intriguing idea of how they could rebuild society. What new world would replace the old one? I am thinking of tinkering with this idea for future works.