Apparently a wall will obstruct a scene full of meshes. So for your buildings room, determine how many players can fit (squished) in the room. Get the total poly count. Now you want the room to have at max, 40-70% of that total number. So, the sum of your vases, tables, chairs, etc should equal that number. Also estimate how many people might be in that room as well. Subtract that from the total and flip it for the number of polys in that room.
this works especially if you know what graphics card will be used and what the constraints are with the amount of visible polys