Just use LOD levels. Every LOD level should reduce polygons at least 50% from previous, and game performance can be adjusted from graphics settings. What causes performance waste is draw calls and texture access. You like to keep number of textures low as possible in asset and read your game engine documentation for details.

Desigining game assets with these guidelines, performance limitations should be roughly your target system memory bandwidth (as you can adjust things). In mobile game amount of storage space is often limiting. Game map fit related assets or chapter should be fit compressed to 150Mb download.

The BlenderBros Hard Surface Game Asset Course ...