I think you're thinking of the Last Glacial Maximum(LGM), 18-23,000 years ago. That was an extremely cold period and this Journey Of Mankind shows an extreme shrinkage of humans worldwide during that period. I've not seen any population figures associated with that period though.
i have a question for you...how does the author know that those lines on the rock are tally marks and not just the grooves left when someone sharpened a stick or arrow head on that rock?
Like many of us, they avoided giving info to the census takers...
;^)