Why do you go with "a human number" (are there "human" numbers and "non-human" numbers?) over and against what Carl has suggested, "the number of a man" = "a man's number"?George F Somsel wrote:What we have in the case of ἀριθμὸς is a predicate nominative "[it] is a human number." If ἀριθμὸς were the subject, we would then be seeking the predicate [in vain].
I agree with Carl that this was written as an invitation for the reader to identify a specific man through numerology (probably some sort of gematria based on the letters of his name adding up to ἑξακόσιοι ἑξήκοντα ἕξ (666) or the alternative form ἑξακόσιοι δέκα ἕξ (616).