Best way to create prefabs for draw calls and performance?

Hey all,

I could really use some clarification on how to best organize some vista card artwork for draw call and performance reasons.

I have a single texture atlas with a bunch of houses on it that are mapped to simple planes in Max and will be used as vista artwork to create a town in the distance. Take a look at the attached image for clarification.

So when you import an FBX file with all the card geometry in there, should I then extract all the cards out and make prefabs of them individually?
Or, should I export all of them as their own FBX and make prefabs of them that way?
Or, should I just use the single FBX with all of them, directly into the scene, and re-arrange them along the landscape? What about adding new cards in the future with this method?

How does Unity determine the draw calls? Is it just by material (in this case, 1) or do assets also have to be a part of the same FBX file to be considered 1 draw call?