I tend to doubt the number of slain Balrogs -- according to
The Book of Lost Tales version of
The Fall of Gondolin -- was going to hold for the later
Quenta Silmarillion.
Unfortunately Tolkien abandoned the 1951 updated (long prose) version of this tale... although at that point Balrogs still existed in great numbers at least (externally speaking). Later Tolkien imagined that, at most, seven ever existed; though in any case he did not revise every relevant passage that implied large numbers of Balrogs...
... just to keep us guessing
