to reinforce frobs statement, for the current time being (pre DX12/Vulkan), Drawcalls will be your bottleneck, not the polygon count.
It is pretty amazing what modern cards can do when it comes to poly count, and game developers are only moving slowly to catch up with their games (for many reasons, one of them being that "modern cards" are actually less widely used than we would like to believe).
Add to that the many advanced culling algorithms that are used by modern engines to further reduce the amount of polygons that will be rendered by the GPU, and the problem of having many polygons in the same level, BUT NOT VISIBLE AT THE SAME TIME is becoming even less of an issue.
So basically, if your polygon DENSITY is not too high, the total amount of polygons in a level is not a performance issue for the CPU or GPU (maybe memory, maybe space on disk, maybe load times... but not rendering)... as long as your culling method is working as intended and you only render what is visible (without choking the system with a too aggressive culling... that can actually decrease performance, if you do too many culling checks without much geometry being culled).
But Drawcalls can choke your system still... as long as your level is built up of too many separate meshes that are NOT combined into a single mesh with a single texture atlas, NOT batched by some mechanism and NOT instanced, each additional mesh will cause additional draw calls. Which is something modern cards still do not handle all that well (its something like a context switch on the CPU AFAIK, thus the old values will be wiped, and new values will be filled into registers and all. Compared to adding some more vertices and polygons to an object that is already rendered at full speed on the GPU, that is way way way more expensive).
There are many ways to reduce the amount of draw calls, ESPECIALLY with something static as level geometry. If you are using Unity, Unity does dynamic batching (no idea if this is now included in the free version). Basically Unity will take all the geometry marked as static and combine it into a single draw call.
You can do the same if you combine all the meshes that compromise your level into a single object in an external 3D App and create a Texture Atlas. This should make the object being sent to the GPU in a single draw call.
And then there is DX12 which should do some kind of builtin batching automatically. I didn't read the specifics, but there where claims that it could create quite large batches out of non-instanced meshes.
Of course, the BIG bottleneck about draw calls is not on the GPU, but the CPU. A draw call is basically the CPU preparing the information needed for the GPU, and then this information being sent to the GPU and the GPU switching to rendering a new object to the screen.
From what I have seen, these draw calls tend to choke the CPU way before the GPU gives in. Depends on your CPU and GPU really, but with a top end i7 2600k and top end Nvidia GTX 680 from 2013 the result was that at least in my case, a prototype running in Unity with too many draw calls, the CPU was choking WAY before the GPU. This was with quite a lot of physics going on, and game logic and other things also running in the main thread.
And as with many other game engines, Unity is still not properly mutli-threaded (Physics AFAIK have their own thread, but the main thread is still doing 80+% of the work).
You COULD get away with more draw calls if you would be able to move the render thread to a different core. Maybe you could split the render thread into multiple threads (as Unity seems to be doing right now with Unity 5.3)... the fact still remains that pre DX12, draw calls are severly limited because they tend to choke the CPU.
Best thing is to reduce draw calls.