Whether to dedicate cities to building units is a decision in and of itself. You can have dedicated cities, or more balanced ones, depending on personal taste and geography. I don't think a percentage really applies. Era doesn't strike me as being relevant, either - circumstances and strategy are much more germane.
The unit question is a two-parter:
You should have just enough units - defensive and offensive - to defend yourself. My preference is for as few as possible, because I don't like to pay a lot for insurance; more responsible souls will feel differently. The more mobile your defensive units, the fewer you need.
Build as many offensive units as you need to accomplish your expansionary goals. The stronger the unit, the fewer you need. How many of a certain unit do you feel you need to take an enemy city, given their defensive units? How many are likely to die or need repair? How many cities do you want to take in the war in question? And how quickly do you want to do it? Answering these questions ought to provide you with your own preferred offensive equation.
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