Easy. As many as you can. I'm of the school that believes each city only needs 12 workable tiles, but plenty of people disagree with me. Maybe your friend only worries about 6 tiles, and you try to prep for Hospitals. Personal preference. But you just can't have too much territory.
Your second question is fine example of condensing the entire strategic focus of Civ3 into 15 words. Short version: I don't. In a war, go whole-hog for the army. In peacetime, do "pushes" -- Cathedrals everywhere to lower the entertainment slider, universities everywhere to maximize science spending, etc.