How many coats of quality paint are needed over old exterior house paint in good condition and of the same color?
None. Why would you paint over good paint?
Otherwise, one. Recent experience with the likes of Behr Premium Plus and other modern products has left me very impressed with coverage and durability.
Of course, this requires good technique. You can't ever "stretch" the paint by pushing it out further than you should. A loaded roller covers something like 10 square feet at most on a smooth surface.