The balrogs are an interesting one. I found this some time ago but just dug it out.
Shows Tolkien revised their number more than once.
Eventually ending with "In the margin my father wrote: 'There should not be supposed more than say 3 or at most 7 ever existed.'"
Having previously written "The early conception of Balrogs makes them less terrible, and certainly more destructible, than they afterwards became: they existed in 'hundreds' (p. 170), and were slain by
Tuor and the
Gondothlim in large numbers: "thus five fell before Tuor's great axe
Dramborleg, three before
Ecthelion's sword, and two score were slain by the warriors of the king's house."
I guess the end state was more LOTR with a balrog being practically impossible to kill for "normal" ME residents
It just highlights the issues with Tolkiens works, they were more ramblings with bits of semi joined up parts into a few books.
When he revised things such as this do you go by the few published books or the more numerous basically unpublished
scrolls manuscripts.
We know many authors and directors will change things at a later date but Tolkien took that to the extreme, mainly I suspect as has been recorded many times, he didn't really have a plan and rewrote parts as he went.