Open the options screen (ALT + CTRL + O) and switch to the Scores tab. Toward the bottom of the screen is an unintelligible string of letters and symbols, something like FSZC?EPRTNBDRH*Q. Yeah, that one. Hover your mouse over the text field and you'll see a popup that tells you the meaning of each letter/symbol. You can rearrange them as you please, double some up, or remove the ones you don't use.
In this case you are asking about Q which is the number of cities that you know about, either by having found them on the map or by opening the trade window and counting the list. When the number is the ones revealed on the map because you cannot see the list in the trade window, the number is drawn in cyan instead of white to let you know it's an estimate.