I inserted edge loops to create the 'windows' but I have now so many faces. What should I do to have a clean topology without changing the geometry?
ALT select edges in Edge Mode, and X - dissolve.
Update: In response to your Sub-div issue. Instead of just removing geometry, you might look at how you are modeling. In the example below I have made the windows edge loops. The one on the left is all quads, and the one on the right has one N-gon in the window - if you don't mind N-gons. The advantage of this geometry can be noted on the sides [top & bottom] where less faces are needed.