I started tagging some images "2girls" if two girls were controlled (not counting non-controlled females in the image), and likewise "3girls" for three girls, but I figured anything more than that should probably be tagged "multiple_girls" if it doesn't fit under "harem."

May come back and do some more later if the weather calms down.

FYI, we weren't going to tag 1girl, 2girls, etc., because it seems pointless. Who's going to search for an exact number of girls? "Multiple girls" is fine, though I would use it for any pic involving more than 1 girl.