FNSKU Code must be put on box or not necessarily?
It depends. Sometimes Amazon requires FNSKU. Sometimes it is the seller’s choice.
If you are not co-mingling your items, then you will need the FNSKU on each item. It goes on the item, not the box you are shipping them to Amazon in (not quite clear which “box” you are referring to).
Co-mingling has some advantages, but many people prefer to avoid it, as you could end up selling inferior condition items sent by another seller.