The Ars Goetia, the first part of the Lesser Key of Solomon, catalogues seventy-two spirits with names, ranks, seals and offices. It is the single most influential demonological list in Western occultism and it is not old.
The compilation, Lemegeton Clavicula Salomonis, is anonymous and seventeenth-century, assembling earlier material of varying dates. Its principal source for the spirit list is Johann Weyer's Pseudomonarchia Daemonum, printed as an appendix to De Praestigiis Daemonum in 1577. Weyer, a physician who was arguing against witch trials, and who published the list partly to demonstrate the absurdity of the material, gives sixty-nine spirits, in a different order.
The move from sixty-nine to seventy-two is the interesting part. The compiler of the Ars Goetia added three and rearranged, producing a count that matches the seventy-two names of the Shem HaMephorash. That matching is deliberate: later Western magical practice pairs each of the seventy-two angels with one of the seventy-two spirits, in a scheme of opposed hierarchies. The number was harmonised to fit a Kabbalistic figure that had itself only been Christianised in the previous century.
So the seventy-two of the Goetia is not a datum received from antiquity. It is an editorial decision, made in the seventeenth century, on a list published in the sixteenth, adjusted to agree with a derivation set out in the eleventh and popularised in Latin in 1517. The chain is documented at every step, which makes it an unusually clean example of how an occult number acquires the appearance of age. Everything in it is real; none of it is ancient.