How many people would be there if only 3 person per acre lives on earths land

12 268 800 000

Explanation:

Landmass in total covers the area of around 57,510,000 square miles. As one square mile covers 640 it means that the whole world covers the area of around 57,510,000 * 640 - 36 806 400 000 acres. So, if only 3 people per acre would live on earth than there would be 12 268 800 000 people in total (36 806 400 000 divided with 3).