Occlusion Culling

Unity imposes no official limits on the size of your scene, and there's no established convention or industry-standard making recommendations about scene size. This is a decision largely in your hands. But there will certainly be a limit in a practical sense. The complexity of meshes, materials, and special effects combine together with the number of meshes and their spread to determine just how computationally expensive a scene is when presented to a camera with a specific frustum and field of view. Unity tries to make scene rendering easier on the computer by applying Frustum Culling automatically. That is, it silently deactivates (culls) objects outside the viewing volume (frustum) of the camera. As objects leave the frustum, Unity ceases to render them, and as they enter the volume Unity starts rendering them. This optimization works well in many cases, for objects that clearly enter and leave the Frustum.

Camera frustum defines the limits of what can be seen

Using...