Considering Dragon's panel above, there are at least 9 for sure. However, I think that there will be more, maybe even up to 13~18.
Why 18? Because I feel that each family has their own "monster"... so 20 families, minus the Nefertari (not CDs), minus the Donquixotes (maybe Doffy was the most talented of his generation, so they missed it out) = 18 God's Knights.
Why 13 as the minimum? That's by subtracting the Gorosei, if we consider that they're already the "top guns" of their families.
Then, there's also the possibility of two representatives of a single family in the God's Knights, as people speculate that Shanks' brother is also a God's Knight. Other families might be the same, so there may be even more than 20 God's Knights.