The minimum amount of extract depends on several variables. A beer needs to have good "drinkability". To achieve that, you need to have balance between the base malt character, the bitterness from the hops, the flavor and aroma from the hops, the flavors from any specialty malts used, the residual sweetness, the alcohol and the flavor profile added by the yeast. So there isn't really one standard answer. Perhaps start with a commercial beer you like and try to find a clone recipe and go from there.