In general the more residents city has, the better. Every citizen makes it possible to put him to work on next tile in city reach or to make next specialist - so, the number of citizens is strongly connected with your global Production, cultural, financial and research potential.
However, you should avoid uncontrolled city growth - too much new people in short time makes part of them unhappy because you don't have means to maintain order in city.
Following factors determine a maximum number of city's inhabitants:
- Food output;
- buildings - Aqueduct and Hospital are increasing health; Forge, Coal Plant or Factory are decreasing it;
- landscape - forest and fresh water helps, flood plains and jungle - cause harm;
- Global availability of Food resources - the health bonus is vital. Every one gives +1 health, and with proper building +2;
- Technological advancements - genetics and future tech increasing health by +3 and +1 respectively.
A little more complex is the technology's influence on Food output:
- Bureaucracy makes possible to build Farms without fresh water adjacent to another farm;
- Biology gives +1 Food from Farms and allow to build Farms even without fresh water and adjacent Farm;
- State property civics give +1 Food from Watermills and Workshop.
As you see, most of these factors are directly linked with city placement, but there are also important global factors which can help a lot - but only if the city has appropriate terrain.
- Temples - Every one gives +1 happy;
- Religion - After you choose state religion, in every city where believers of this particular faith are, you will have +1 happy (and +2 culture, what is usually also very important);
- Culture - With such buildings like Colosseum or Theatre culture generated with your Global Settings (more about this in chapter Money) greatly improve citizens' happiness;
- Luxurious resources - every one gives +1 happy, and with proper building gives another +1.