From the website Torahresource.com
Here is wisdom. Let him who has understanding calculate the number of the beast, for the number is that of a man; and his number is six hundred and sixty-six.
Rev. 13:18 Ὧδε ἡ σοφία ἐστίν. ὁ ἔχων νοῦν ψηφισάτω τὸν ἀριθμὸν τοῦ θηρίου, ἀριθμὸς γὰρ ἀνθρώπου ἐστίν, καὶ ὁ ἀριθμὸς αὐτοῦ
ἑξακόσιοι ἑξήκοντα ἕξ. (666)
(א, P, 1006. 1841. 1854. 2053vid al)
(A, P47 051 M; Ir Hipp)
ἑξακόσιοι δεκα ἕξ. (616)
ἑξήκοντα πεντε (665)
Neron Caesar (Greek)
Nero Caesar (Latin)
The other numerical sums, found in a few manuscripts (e.g., manuscript 2344 which has “665”), are probably scribal errors or perhaps were done by scribes who misspelled the Hebrew transliteration of the Greek or Latin name Nero Caesar. If, for instance, Nero was spelled הרנ rather than ורנ, the numerical sum would be 665 rather than 666.