From an AskScience Reddit thread:
But keeping to the hypothetical, a fertility rate of six with 4 surviving to reproduce themselves on a similar basis would lead to exponential population growth; the survival of the species would be pretty much assured if they could keep this up for only a few generations simply due to force of numbers. A couple having four children survive to adulthood is a doubling of population each generation.
As a purely mathematical fiction, presuming an average generation gap of 16 years and that this average of four children per couple surviving to reproduce held solidly (it wouldn’t, of course) it would only take a little over 500 years for this original couple to have repopulated the earth to the present level of 7bn (that’s exponential growth for you.) Of course plenty would happen in between to slow the exponential growth.
Assuming no wars, famine, disease – yes, and in only 500 years. Of course, human history shows that many things do get in the way of humans rapidly multiplying like we theoretically could.