Answer:
So God blessed Noah and his sons, and said to them: “Be fruitful and multiply, and fill the earth” (Genesis 9:1).
First of all, there are now well over six billion people in the world! I’d say Noah’s descendants did a great job of filling the earth. According to the Bible, the Flood was about 4,300 years ago. So, can we account for six billion people starting with only eight just 4,300 years ago? Here is an example for you: Say you get $1.00 on your first birthday. Your parents say they will double your gift on each birthday until you are 21. When you’re two you’ll get $2, when you’re three, you’ll get $4, when you’re four, you’ll get $8, and so on. You just double your birthday gift from the previous year. Well, when you are 21, your gift would be over $1,000,000! Population growth is similar to that. Given enough generations, the number of people being added with each generation would be huge! Just like each of your birthdays above started with more money, each new generation starts with a lot more people. When you do the calculations, you will find that it is very easy to account for six billion people starting with only eight after the Flood.
Source: https://answersingenesis.org/kids/noahs-ark/how-could-noahs-children-fill-the-earth/