Chris says that Tolkien did indeed settle on a definitive number- which I cannot remember off the top of my head but I have posted in this area before, for anyone who wants to find it. I have a sneaking suspicion it was about 12- regardless, it was very few. And that being the case all earlier stories about Men killing several in one fight were rendered very much out of continuity, being written when Balrogs were far more numerous and more like large fiery trolls than the demi-Gods they had become by the time he wrote LOTR.