There were giants on the earth in those days, and also afterward, when the sons of God came in to the daughters of men and they bore children to them. Those were the mighty men who were of old, men of renown. According the names and years, listed in Genesis 5, there are about 1600 years between the creation and the flood. This gives some indication about the population on earth when the flood broke out. However, there are some things we cann't say for sure, of course.
Some questions remain.
Suppose all people lived about 600-700 years. They all were marrying and girls and boys were equally divided. Suppose each family did have 20 children. This doesn't feel like just a handfull of children. And suppose there were only 10 generations between the creation and the flood.