Apparently the covers used in rentals are different from the retail versions. Jean and Marco (rightfully) shares volume 4’s; though now Connie’s been bumped to volume 5 to be Sasha (also just as fitting!). So very likely Historia and Ymir get shoved to volume 6..
To get a sense of the popularity, take a look at the oricon chart back in October. Single was released back in July, and it managed to stay within top 30 for 3 months. (a rather difficult feat to do so in oricon singles chart imo)