The ZFS Deduplication | TrueNAS Documentation Hub [1] states between 1 and 3 GB… 5GB per TB of dedupled data is suggested to avoid the nasty situation of having the DDT (deduplication table) outside your RAM… usually meaning on HDD; it however depends on the average record size![2]. None should be that guy.
I really suggest reading it a few times, it’s a GREAT resource! ↩︎
Resource - ZFS de-Duplication - Or why you shouldn't use de-dup | TrueNAS Community same as above. ↩︎