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I think, and on SO it's a common though, that a tag should be in plural form if it can and that doesn't change sense.
So let's pluralize minifigure by analogy with video-games.
minifigure is now synonymous with minifigures which is now the primary tag.
It looks like nearly every other tag that can be considered quantitative has already been written in plural form, it seems sensible that minifigures should be treated the same.