Only show some faces of a cube

How can you only show the faces of a cube visible to a player?
More precisely: Let’s say you have a 5x5x5 block of cubes. You don’t need to render the whole thing - because that’s eating verts, particulaly as you get bigger. So how do you only show the faces which the player can see, and all the others are ‘non-rendered’, and only come into being when the player could view them.

It sounds like you’re asking about occlusion culling.

U could try Occlusion Culling:
http://unity3d.com/support/documentation/Manual/Occlusion%20Culling.html