Yep, quite a few were killed. BUT: when Tolkien wrote this, Balrogs were different from the one in LotR, because they weren't as powerful, as they weren't Maia. Later Tolkein said that there would be only 3 or 7, at most, Balrogs, becuase if you had armies of them, as there were in BoLT, and they were as powerful as the Balrog of Moria, then they would taken over the world.
