We've got no reason to think that the population would be any different than it is now. Although it would likely be rather lower if the young earth creationists were right.
The creationist models are usually based on simple exponential growth without considering other factors - notably "carrying capacity" - the population that can be supported and how technology affects it. Typically they do not consider historical population figures for a large part of the time they cover. As such their models are not only simplistic - they have little contact with the evidence. As a result, they are worthless.
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