Five! No it really depends on the object. For a closeup of a gun barrel you would use more sides then for the pillars of a fence. General rule of thumb is, if a player can get really close to a object and its a hero-object then add more polygons to it.
There is no hard rule. The minimum amount that looks good for the art style or the technical constraints of the game. Example #1: The trees are 6-sided cylinders. Example #2: The valence count of the cylindrical parts is exactly as high as it needs to be in order to eliminate a jagged look in the silhouette at the closest…