I'd say it depends on how big you want the windows to be. Are the buildings going to be a ground row? Supposed to be at the foot of the building or more of a skyline kinda far away?
I've found that just the shape of a building, then christmas lights poked thru work great. I made a NYC skyline and had cut out the windows, about 2" square, but really only the lights gave the impression - not the holes. I could have saved myself a lot of work bu drilling christmas light size holes rather than cutting out the squares.
Kenneth Pogin
Production / Tour Manager
Minnesota Ballet