The explanation of the plural - “sepulchres” - will be seen in 1 Kings 13:30 note.
The burning of spices in honor of a king at his funeral was customary (compare the marginal references).
Related Commentaries of 2 Chronicles 16:14
2 Chronicles 16:14
14 And they buried him in his own sepulchres, which he had maded for himself in the city of David, and laid him in the bed which was filled with sweet odours and divers kinds of spices prepared by the apothecaries' art: and they made a very great burning for him.