I'm not quite sure what you mean by "mobj number," but I'm assuming you're looking to be able to modify rings that are spilt by the player? I've done some work with spilled rings in the past so maybe this will help.
Hooks like MobjDamage execute as soon as P_DamageMobj is used, which happens to be before the rings are lost—so if you want to find the number of rings the player will spill, simply find player.health - 1 in a MobjDamage hook for MT_PLAYER. If you instead want to modify rings once they are spilled, you can use a MobjThinker hook for MT_FLINGRING. They conveniently set their target to the player that spilled them, so you can use mobj.target to find the player that spilled that flingring! MobjSpawn might be more efficient depending on what you're trying to do but unfortunately that occurs before the ring sets its target to the player, so its use is limited.
Hopefully that answers your question, but if not could you be a little more specific? :)