I killed 6 volatile skeletons on my opponents board at the same time by casting remorseless winter. Five of the volatile skeleton deathrattle triggers hit me in the face for two damage each but my equipped arcanite ripper only improved once. I did not have armor or anything else that would have prevented my health changing.
It’s probably because they all triggered in one instance/chain. Its the same way how blood boil only increases it by 1 whether you use it on one minion or five.