When you start a new game the map is small. Most new players, or even veterans, will forget about the maximum city range. Even the game will try to misinform you of optimal city placement! Cities will eventually overlap. Since you cannot move cities you are usually better off with optimal spacing! In this article Gamplayinside explains all about city spacing.
In Civilization V optimal city placement is key! You might think spamming a few cities does not matter but each city you found or conquer has a big impact on your game. This gameplayinside article will show you the worst spots to build cities so you can avoid making mistakes!