I've always felt that the best way to start a discussion on a topic is to have a understanding of the topic itself. So we're on the same page, I've visited the Marion-Webster website to grab a couple of definitions:
Topography: the art or practice of graphic delineation in detail usually on maps or charts of natural and man-made features of a place or region especially in a way to show their relative positions and elevations
Terrain: the physical features of a tract of land
Battlefield: a place where a battle is fought
So, our whole purpose will to be to recreate, in a small scale, of course, the topography and terrain of the battlefield - or at least my purpose will be to do so :P.
Now, having said that, the terrain you build for your gaming will depend on the game you play. The terrain for a game played using first century Romans and Persians will be much different than the underhives of Warhammer 40000 - as will the chosen battlefields, in my opinion - although a lot of 40k battlefields I've seen could be used for Romans as well - flat tabletops with a few hills and trees to break things up.
A second consideration should be how big a playing surface you have - if you are playing small tight games on a small tabletop, you need less terrain pieces than you would for a larger table. In addition to playing surfaces, you need to consider storing your terrain as well - the more pieces, the larger the size of said pieces, the more storage space they will take - which should be a consideration.
With all that being said, where do we begin?