[obloquy]

Only three Balrogs need to exist: Durin's Bane, Gothmog, and Glorfindel's Bane. These are the three that receive solid canonicity by being included in writings that Tolkien himself saw published. Tolkien reduced their early multitude, as has been mentioned, and indicates that he might have removed all but just these three principals ("3 or at most 7"). This adjustment ought to be seen as an implemented change, since none of the texts that conflict with it were ever published by Tolkien himself, and were, per his written intention, obsolete with particular regard to balrog numbers. The Fall of Gondolin has almost no bearing on balrog discussion.

Edit: it occurs to me that Gothmog may not have had his canonicity cemented as the other two, but I don't have time right now to check. In any case, there's no indication that Tolkien intended Gothmog's fate to change.