Number could be more than 8 billion, because
Flood story is a poor copy of the myths found in other cultures. Essence of those myths is that what is new becomes old in the due course of time, and God renews what has become old through some means which varies from culture to culture. That means Old World would end with more than 8 billion people, and New World would again start with two people.
Hebrews preferred a global flood which may or may not be true. What is true is that Old World would end through some means, and New World would begin with two persons. This would mean there are two phases to each block of history [and two different words for humans to describe their pre-fall and post fall nature]:
First half of history [New World] is filled with perfect people who had control over their body organs.
Second half of history [Old World] is filled with imperfect people who lost their control over their body, hence began to be ruled by body.
Interestingly, the word diamond comes from the Greek adamao, which means "I tame" or "I subdue;" and its adjective adamas means “unbreakable.” From this came the English word adamant [from a = ‘not’ + daman = ‘to tame’] meaning “untamable, invincible”. New World people are adamant stubbornly refusing to listen to Satan (vices personified). In Hebrew language (older than Greek), related to adamah is the word adam, which means “man” or “mankind” in their sinless stage during pre-fall period when they had perfect control over their bodies. In the East, the word for man [admi, admah ..etc] has the same connotation. The Sanskrit words "manush" "manava," meaning “a person with thinking ability,” have the connotation of one who is in control of himself acting thoughtfully in consideration of pros and cons before executing the action, thus using his thinking abilities to rule over his bodily organs.
This is in contrast to the post-fall people of the second half of the history [also called narak or hell] where they are being ruled by body [bulk of which is neer or water], hence humans are called nara-naaris [from neer, water] who show the characteristics of neer or water that takes the form of its container, the symbol of being ruled by circumstances.