Most of the countries are founded on s territorial claim. There are also a country equivalent that doesn't have any land and another one that is in danger of losing all of it. Then there are numerous cases of individuals or groups claiming tracks of land and proclaiming them independent. So far none of these micronations have gained any official international recognition.
Land is such a sought after resource that almost, but not quite all of it has been claimed by all those sovereign states. There are huge areas of land in the Antarctica that doesn't belong to any nation and which most of the countries have agreed to not claim. There are small areas which ownership is unclear. Like the river pockets in River Danube between Croatia and Serbia.
The only real terra nullius, nobody's land is located between Egypt and Sudan. Bir Tawil is an area of a bit over 2000 square kilometers that no country has claim over. This is a result from differences in claims between these two nations in the break of the 19th where neither of them claimed the area. Instead they both did declare the claim over the Hala'ib Triangle area next to it.