How many cities? As many as you can get and defend, as long as they are valuable (good commerce or production) or give you access to an important resource. You just have to keep maintenance costs in check, which means having a well placed capital / forbidden palace (or state property civic) and courthouses in every city. Costs go up with city distance from capital and population. It also depends on map size. You don't want to lose too much ground to the AI, so you need to expand before they get all your surrounding good land or they turn into super powers with double or triple the number of cities. One thing to be wary of is settling cities before you can afford it. Don't go crazy before you have access to courthouses, and don't settle if you are already low on treasury and running 0-20% research. You might even want to prechop forest in the area (if under borders), then settle the city with a fast courthouse. Also, do not settle more than one city on the same foreign continent, as colonial maintenance runs high. Corporation expenses are also significant (but usually worth it). Coastal cities can usually pay for themselves through trade routes (civics / buildings / open borders / great lighthouse / population / ). Inland cities need commerce tiles from resources / cottages or could run specialists. Otherwise, they'll need to be paid with money coming from other places (a good production city or great people farm is usually a worthwhile investment, even if it doesn't provide much commerce, so is a city giving you access to that single copper/iron/horse/ivory/coal/oil/aluminum resource, or even luxury/health ones you don't already have). I believe 10 commerce would usually offset the costs in a city. Build lots of commerce cities (cottages or farms with specialists, you need to choose a strategy and synchronize you cities and civics). When in trouble run some merchant specialists. And don't forget to train enough workers. Unimproved land isn't very profitable, it doesn't hurt to improve some of the land before you settle. Lastly, keep cities connected through rivers, roads, coastlines. (settle once you have the spot connected) Remember to chose good city spots with a focus on food, even if it means wasting a few tiles, or overlapping some borders. There's a lot more to be said... but that's all I've got right now.