In Tolkien's earlier writings, there were hundreds of Balrogs; later on he changed that to there only being seven of them. One way to split the difference could be that the seven were the survivors who hid in Sauron's cellar while the others all got killed. :)
(Reminder - 'Sauron' is what Mairon started calling himself in order to sound more cool and badass.)
Part 57: Run away!
Next time: Part 58: Mopping up
Chibis by tektek.org