Try to keep Number of objects as little as possible. Under 300 max.
Merge what you can merge. If you have many individual objects and not intend to make them interactive (e.g. bunch of “Clouds”, “Buildings”) - try to merge and keep them as one object with 1 material. The method you use to construct objects can have a massive effect on the number of polygons, especially when not optimized. In this diagram, the same shape mesh has 156 triangles on the right and 726 on the left.
Optimize your geometry if it has too many polygons. Many character models need to be intelligently optimized or even rebuilt by an artist