Hi publish game makers

I was talking with 3 artist 3d modelrs who had a interest in working in this community. They have questions about ploy limit for models.
max limits for a single model
determining limits for polys based on number of objects in scene
how to correctly determain lod poly steps .
Wither the engine uses facades" - some idea where the engine determiners the sides of builds that are visible , visible sides are mixed at full lod with none visible sides at lowest lod and then the clipping away of none visible ploys is fast because less ploys need to be clippd - I really didn't understand to well this question they asked