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”.
In Aramaic, 60 is štyn (instead of Hebrew ššym), so it puns with “Satan” & “opposition”. It is likely an important encryption pun number, because very often the Aramaic term for “sixty” is used in the Talmud.
The Satan character in the Old Testament is a pun with being an “opponent” for Job case in the heavenly court, and generally being a type of “hostile” spirit, as the word Satan means “opposition” & “hostility”. In the Old Testament he isn’t yet particularly evil though. Satan’s modern use for scaremongering was probably kickstarted in later times, by cobbling together a bogeyman with nonsense 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 numerological synonym 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