You might feel like you can rule the world with just one mega-city? You would not need to handle dealing with numerous cities specifically throughout times such as insolvency or war, you would not need to hang out putting resources into the new cities, etc and so on. It is recommended to build at least 2 more cities besides the Capital that you start out with. The factor for this is managing several cities is not very challenging. If you keep your cities close together and not too expanded, you can practically have a string of cities that seem like a single, large city. When it comes to the resources, new cities do not use up that numerous resources and they also enable you to trade in between your own cities once you have multiples, returning resources that you would have invested in producing the cities. They permit you to spread your judgment area and also provide your Capital an additional layer of defense throughout times of war if you are going for a Domination Victory. There's very little disadvantages to building cities and numerous advantages to doing so.