A few features help you manage the polygon count for scenes and animations that must not become too complex.
Show Statistics, available from the
General viewport label menu 
xView_submenu, lets you monitor the number of polygons, vertices, and so on, in the scene, as well as the frames per second displayed.
The Level of Detail utility, available from the Utilities panel, lets you manage the complexity of an object in the scene. For example, Level of Detail enables you to display a complex object as simple geometry when the object is at a distance from the camera.
ProOptimizer is a modifier that helps you reduce the vertex and polygon count of high-resolution models. This feature is also available as a utility, which lets you run it as a batch process on one or more scene files.