To be precise, a distinction was made between Imperial Cities (Reichsstädte) and Free Cities (Freie Städte); the latter was mostly used for cities formerly governed by a bishop that had managed to gain independence to a degree comparable to the Imperial Cities during the High Middle Ages.
The number of Imperial Free Cities varied greatly over the centuries, but rose into several hundreds; however, at the 1792 Reichstag, a mere 51 cities were left bearing this status (see List of Reichstag participants (1792)). Free and Imperial Cities were only officially admitted as a Reichsstand to the Reichstag in the 15th century, and even then their votes were less significant compared to the Kurfürsten (Electors) and other dukes.
The most powerful Reichsstädte included Augsburg, Bremen, Frankfurt, Hamburg, Lübeck and Nuremberg. In the southwest, which had a more diverse and scattered political structure, many more free cities existed than in the north and in Bavaria, where larger territories had established themselves.