How confident are historians when they conduct studies like this predicting entire world population eons ago. I can't wrap my head around the fact that most studies estimate that only 2-4 million homo sapiens existed 12,000 years ago. Is there any possibility that these studies are way off?
You would be better off asking this question in /r/AskScience or /r/AskAnthropology, because those estimates aren't based on historical records or recorded history, but they are based on scientific analysis of cultural artifacts and such, which is different. Not that there weren't any historical records 12,000 years ago, but something like world population estimates aren't in them nor could they particularly be parsed out based on historical records alone. You'll need to use some fancy scientific know-how to make such estimates, which is outside the scope of the discipline of history.
A scientist or anthropologist would be able to better answer this question for you.