I think it is not that sure of a prediction. It is still at least 2.5m sold, so basically some 9-10m in total for Destiny over 4 platforms approx.
That will be really tough, even with all the hype, it's still a new, always online game.
I think that Activision would be happy with 10m, though.
My prediction is that PS3 > X360 version, due to the fact that it's the only version playable without a subscription.