Theoretically it should be possible to carry one on the centerline as well right?You are asking about quantity, when the question should really be about total mass.
YJ-12s are heavy.
If they develop multi ejector racks and a new smaller (but inevitably less capable and lower payload) missile, then yes they can carry more.
If you are asking if they can carry more YJ-12s than this, the answer is no, and it's been known for nearly a decade that the H-6K/J airframe is only able to carry 4 YJ-12s and only on the innermost 4 stations.