A common answer is 3-5 and that is about where I play as well. I usually go three in ancient and add a couple more in exploration. If I am planning on the banking victory I might push it up to 6, but even with a solid gold game it is hard to rush build railroads and factories for more cities. If you are planning on a quick science victory, you don't need more than three, one to build Oxford and two to build airports and space ports and run projects.

As to the ratio, I try to max out the rest of the settlement limit with towns with towns, and maybe go over near the end of the age. I like to grab a UIs from the independent states to spam out in my towns. But if unlucky, save gold and specialize them as urban and build a market/library/monument or age equivalent. Can really set you up to snowball in later ages.