(Civilization 5) Why should the number of R&D rounds increase in one more city?

The more cities, the better, but the bigger the better. To build a new city, the number of research sites needed for scientific research projects and the number of cultural sites needed for the new opening policy will increase by a percentage. In other words, if you can't buy a building with scientific research and culture, basically opening a new city will delay the development of your technology and policy. This delay is not obvious in the early stage, but it will have a great influence in the later stage. However, this does not mean that there is no need to open new cities, because the more cities, the more money. In peacetime, there is no need to keep a large number of troops. Once the war begins, you can quickly enter the battle by spending money to buy units. With money, you can buy buildings for the city and promote its growth. It should be said that the conquest needs to properly open Kaesong, and the cultural and technological victory needs to avoid opening redundant cities, as long as the cities can maintain a world-average military unit. Moreover, Kaesong does not want to open, but mainly grabs resources, or other cities develop to a certain law. In a word, building a city is a long-term investment with limited short-term returns, which will also take up your funds. Your vision should be long-term, and the location of the city should be good, which is conducive to the later development.