There is no rule and you would add what would be appropriate for the beer you are making. Yes, a lot of extract recipes do often work out to about 1lb of DME per gallon, and that means total gallons not pre/post boil (eg. 5 gallons and 5lbs DME). But again, it depends on what you are trying to achieve - ie. Gravity, SRM, ABV etc. Heavier beers with more alcohol will require more extract for example.