How much unhappiness is generated from cities and citizens?

You'll want to check out the resources and happiness tab on the Economic Overview, since the exact formula varies depending on map size and culture bonuses. It also changes sometimes in patches.

The base formula as of this time is 3 per city, and one per population (Or 1.34 for occupied cities). You can also find/change these values in the GlobalDefines.XML file.

UNHAPPINESS_PER_POPULATION 1

UNHAPPINESS_PER_OCCUPIED_POPULATION 1.34

UNHAPPINESS_PER_CITY 3

The modifiers for map size are found in CIV5Worlds.XML. As of this time, all levels use the values above except for Large (80% of the above values) and Huge (60% of the above values). This affects both population and city unhappiness equally.

In terms of evaluating Gandhi's ability, keep in mind that the maximum happiness you can gain from a city is equal to its population. So for anyone else, you can never do better than -3 happiness per city (without culture bonuses). For Gandhi, you start at -6, but at 12 population you could theoretically have a net 0 happiness from the city. So Gandhi can theoretically have MORE cities than others, even though the intent is to encourage you to have fewer.