The tile "filled" by a City is the 9th upgrade of the planet and its villages. It might support many people and humans, as therefore are 1.000.000 of PP needed to afford its upgrade an develop it, and maybe it don't need the upgrade... or does? But well it is good to have.
It remains, stay in the planet (accept itself) while there are still some Villages and many Humans existing in the average tiles of the planet. It must have many Civility points and may need a good amount of Humans in this tile for it to develop. .Also the upgrade itself have the following:
. . Cost: 1M PP (first evolution of the upgrade) . . First Upgrade Capacity (FUC): 21k . . Avaiability: at Semi-Advanced Ages in Gameplay (more than 1 day in gameplay at least to achieve/unlock feature) . . Upgrade Menu Label: 9th position (from up to down)
It seemingly can house many people. Without its respective upgrade it needs Civility in HUGE amounts and a lot of Humans in its tile(s) to get developed, and can in a quantity smaller than 13 (roughly half of 27, value of upgrade). It get much easier and the cities develops each time faster and in more tiles as the upgrades advances. It holds while Villages stay in the planet and a very good amount of Humans and many, many, many Civility points in the average of all tiles of the planet. The following is the theoretical formula of where the cities develop, assuming before the 1st upgrade the upgrade level for City is 0: n2 > n1; n1 ≈ (((((n4 3:1 n3) / 2.8 ) * 1.94) * n2) - (n2 * 1.82654)) * 8.5 where n2 is the upgrade level and n1 an integer with the approximate rate of city development per tile per (rough/average) world. n3 is the average Humans and n4 average Civility, both through the planet. Note the "n4 3:1 n3" in the formula; it is justa one proportion where for a city to grow in ideal conditions, "there must be 3 points of Civility for each 1 Human" plus every other detail in formula. Also the result of n1 will only work if the cities it generate (the variable n5) is smaller than the City upgrade's capacity (n2) (being the FUC the 27 cities in the planet) in the whole planet.