There are two very conflicting schools of thought for number of cities.
On one extreme, there's OCC, and on the other extreme, only build the bare minimum buildings in each city, then crank out units non-stop, without pausing for even workers, relying on capturing workers and leaving improvements as they are. Of course, what's "bare minimum" can vary tremendously.
For OCC, you're effectively creating cities in your one city by settling great persons, and highly efficient too, because of all the bonuses you get from multipliers and wonders. As long as you have health resources and enough to trade around, you should blast ahead in tech early and will eventually last through to a diplomatic or space if your spot is great.
On the other extreme, here's what I consider building if you want pure warmonger, cranking out units non-stop:
- nothing but courthouses in all cities
- libraries in some
- barracks in most
- granaries in very population-devastated cities (high homeland unhapiness, but then in large cities granaries are usually standing anyway)
- theatres if religions somehow doesn't spread there for a long time, and libraries are not worth the hammers. Also hippodromes if you're Byzantines, as you can draft like mad and simply turn on the cultural slider 40%.
And that's is IT.
Your slider will be low, you won't have a good great person farm, and you can pretty much forget about national wonders. However, it works really well because in Civ, having twice as much land means having twice as many luxury resources, which usually ALL your cities are larger. You don't need things like colosseums and aqueducts, as your sheer variety of resources will sustain your underdeveloped empire.
For a builder-biased game, I would still build the minimum number of cities needed for national wonders, and for an expansion-biased game, build a city as long as it either gets you a new type of resource, or if the spot is good enough such that MB>MC (check your maintenance costs).