Well, Izuku is the ninth wielder, you can see four people, five if you count the first wielder (AfO's little brother), two are still in shadow, and the yellow one is undefined, but clearly a representation of All Might. Nine wielders of OfA. If I remember this right, AfO gave his brother a quirk that could stockpile power because he was so frail and sickly. But that wasn't OfA yet. The little brother had a quirk already that he didn't know about, the power to pass on quirks. The two quirks merged with each other and the end result is what we know as OfA, so the little brother is indeed the first one to have it.