I think SNSD will probably end at #1 for 2016, but some other groups like EXID, AOA & SISTAR aren't doing as well this year.
Gfriend will do at least what they did last year and TWICE will probably triple theirs (1.5m so far for both of them), and IOI, a new group, have sold about that much as well.
All in all, it's quite a mixed bag, so the last half of the year will be what defines it.