If all 4 couples who came through the Flood were reproducing and had a baby every other year, and if individuals married at 18 or 19 and also had babies every 2nd year, I calculate that after 50 years there could have been 553 individuals, including 76 couples; 3,139 individuals, including 407 couples after 75 years; and 19,719 individuals, including 3,381 couples after 100 years.  I suspect, however, that the four couples aboard the Ark likely had small children with them, which would have enabled the post-Flood population to have expanded more rapidly.