ἑξακοσίων (Adj-GMP), Revelation 14:20
ἑξακόσιοι (Adj-NMP), Revelation 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.

hic sapientia est qui habet intellectum conputet numerum bestiae numerus enim hominis est et numerus eius est sescenti sexaginta sex

Here is wisdom. Let him that hath understanding count the number of the beast: for it is the number of a man; and his number is Six hundred threescore and six.