^^ Using flat shading and sharp edges will increase the vertex count in Tomb Editor, because you can only have sharp edges in the tomb engine by splitting the vertices, doubling and trippling the vertex count. Cube = six faces with four vertices -> 6 x 4 = 24. You can switch between flat and smooth shading by selecting your model in
Object Mode and right click:
^^click for bigger view
While smooth shading is best for vertex count it's not always the best looking option; some edges just need to be sharp and the improved aesthetic might be worth the increase in vertex count. In that case it's best to set your model to
Smooth Shading and then select those edges that you want to be sharp in
Edit Mode and mark them as sharp:
That way you can control which parts of the model will be smooth and which edges will be sharp.