nonamethanks said:
Following our typical naming convention for this kind of tags, multiple mouths is for any character with multiple mouths, while extra mouth is for any character with more mouths than usual. A character normally depicted without any mouth but having one in a specific post would qualify for extra mouth, but not multiple mouths. Likewise, the opposite would apply to a character that normally has more than one mouth and is depicted with the same amount of mouths they have canonically.
I'm kinda lost here, because if "more mouths than usual" means "more mouths than they usually do", then no, that's not the usual tag usage, see extra ears, extra eyes, extra arms.. in fact, I think the only tag that follows this is extra tails
And if it means just more mouths than a usual human, then the second bur would be correct
Your point about drawing a mouthless character with a mouth being extra_mouth is valid, but these are more often than not a stylistic choice.
Plus the term extra almost implies plurality (Oxford defines it as "added to an existing or usual amount or number").
One wouldn't tag extra_tail on a post with a character that doesn't have one usually (i.e. kemomimi mode), same with extra arms on a usually armless character