People are guessing (details haven’t been released as to what the contents are)

9 4k UHD discs (one per film)
9 Blu-ray discs (one per film)
9 bonus discs (one per film)

And that seems to be the best bet considering the blu-ray only set is exactly 9 discs less than the 4k set.

It could be possible that the 9 bonus discs are not a per-film thing (this is where whatever original versions hope one might have still lives) at which point you could have the bonuses broken out in a bunch of different ways, with three discs (or even more, considering the theatrical cuts of the Prequels aren’t available anymore either) dedicated to presenting the originals at 1080p, and all the rest of the bonus features occupying whatever discs are left.