Very small numbers. The Nazis scorched the earth all the way back from Stalingrad to the Polish-German border, leaving very little of military (or even civilian) use intact.
Minsk, for example, was recaptured by the advancing Soviet armies extremely quickly during the collapse of the German Army Group Center in the summer of 1944. Yet the Nazis still had time to demolish the city, leaving less than a dozen buildings standing.
Brad DeLong