Recently I have been doing some versions for Necromancers for one shot and I have been struggling to find something that mentions how to keep up with all the corpses you will need for Animate Dead Spell and Create Undead Spell. .
It seems that there is an indefinite period in which a Corpse can remain as Corpse within the 5E RAW.
Nor have I found anything that indicates that the living dead that die again can not be animated again, within RAW.
This implies that once my zombie or skeleton dies and becomes a corpse again, can I reuse the now corpse according to RAW?