Seraphim are new Pacifista units developed by the Marines’ Special Science Group with the intention of replacing the Seven Warlords of the Sea as one of the Three Great Powers. They are living weapons built in the childhood likenesses of the former members of the Warlords, but with the traits of the Lunarian Tribe.
We’ve seen four Seraphim, and they seem to be all that there are.
The Jinbei Seraphim is numbered as being the 4th and appeared to have only just been completed when the Straw Hats ran into him. This suggests that there aren’t any more Seraphim and that if there are other Seraphim they are not finished yet and won’t be, since Vegapunk is leaving.
The current 4 Seraphim are made from the Shichibukai who were, if not the most loyal, the ones that caused the World Government the least trouble up until the Summit War.
The World Government themselves did not consider the abolishment of the Shichibukai until the time-skip and the abuse of power and quick turnover became an issue.
The Shichibukai were not even questioned as being problematic until Crocodile was defeated and his whole shady enterprise was exposed.
Moria was removed as a Shichibukai for being too weak, so making a Seraphim of him seems like a bad investment.
Making a Seraphim of Doflamingo, a former? Celestial Dragon, seems as though it would be some kind of in-world blasphemy, given the no doubt Void Century connotations.
From a Story perspective, it stands to reason that the Seraphim should only be of the 4 undefeated OG Shichibukai.
Theory by OnlyM34