Technically, earth's land area is big enough to sustain around 24 billion people. Consider this diagram:
It shows that we're using around 50% of all habitable land for agriculture. Most of the land that we aren't using is either high up in the mountains (where terrain isn't flat and you can't use heavy machinery) or in the tropical regions on Earth close to the equator (south america, central africa, indonesia), or in areas where it's too cold for agriculture (sibiria, canada). so you can't really use more agricultural land than we're already using without cutting down the rainforest.
In the diagram it also says that we're using only 23% of agricultural land for crops which produce 83% of all calories. If we used close to 100% of agricultural land for crops, it would produce approximately 320% of calories currently being produced, so yes, we could feed 3x the population this way.
However, it must be noted that there's significant fluctuation in food production per km², for example due to volcanic eruptions. So it's better to leave a certain buffer to the maximum amount of people you could feed in one year, because food shortages in another year would otherwise lead to bad famines.