I use Unity. The general rule is the lower the polycount the faster it will render, the better your game will run.

But you can also get around that by having fewer objects in the scene or streamlining your code etc.

And if you're developing for mobile vs PC vs game console there are different rules. Developers tend to develop for multiple markets simultaneously. For example, if I'm developing for mobile and PC, my assets must work on mobile, but still look good on PC. Alternatively, they'll use version control systems to duplicate the assets - select a low poly asset for mobile and a higher poly count for PC. 

Things should be in pieces - rather than a room full of furniture, you'd need a room and furniture. The developer will select what they need for each platform, based on what that platform can handle (mobile will have fewer items - pc will have more items).

Look at the unity and unreal asset stores and you'll get a good idea of what's popular, and the polycounts.

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