Number 6
The number 6, especially in its derivation number 60, puns with “opposition” and “Satan” in Aramaic. Number 6 was made the “number of Satan”, simply because in Aramaic it is pronounced št like “Satan”. The Biblical character named “Satan” was originally the judical “opponent” & “accuser” in Job’s story, because both Satan’s & Job’s names mean “opponent”.
A giveaway is that very often the Aramaic term for “sixty” is used in the Talmud, which is štyn instead of Hebrew ššym.
In some instances, authors even write “opposition” like Aramaic “sixty”, with Tav instead of Ṭet.
The Satan character in the Old Testament is not a particularly evil “opponent” though. His use for scaremongering was kickstarted in later times, by cobbling together a fairy-tale bogeyman with features that all pun with “Satan”.
For a list of Satan puns (which all pun with Aramaic six & sixty as well), see the full Satan entry.
By contrast, the number 666 is not a homonym pun, but a spelling pun: It spells trs, which is a synonym for Satan.
Hebrew, Aramaic štn = sixty; šṭn = opponent, Satan
…He announced it for sixty, and it was worth fifty…
qry štyn wšwy ḥmšyn
קרי שתין ושוי חמשין
If the wicked ones arise and destroy the foundations which face the deeps, the righteous of the world…
ˀm hšytyn šhm rwˀyn ˀt hthwm ˁmdw ršˁym wpgrwm ṣdyq hˁwlm wkw
אם השיתין שהם רואין את התהום עמדו רשעים ופגרום צדיק העולם וכו