From what i can see from the picture you have used a Lambert material for the entire model. A Lambert is a flat shaded material with no specular highlight and to create a metal you would certainly need at least some spec.
A blinn would be a better starting point for your texture and then i would think about creating a specular map so the highlights are not even over the model, in the real world not every piece of the robot would be made of the same material so it follows that some parts would be shiny and other less so.

with regards to your question about the amount of maps..... there is no real limit other than time constraints and RAM, every texture map you add will increase your render time and the larger the texture resolutions the more time it will add. For a still image such as this that should not really be and issue here.
You could increase the resolution of your maps for more detail or use more maps with less resolution.

One last tip for you final render i would try and make the lighting a little more dramatic and or even the camera shot position since right now the lighting is very flat and even and the composition od the scene is not the most exciting.

I hope this helps you a little and good luck with your grade.