As Daiske said it isn't fixed. I used approximately 133 seal scrolls to pull all three of the rares from the pack. Oro came at around 40, Ao took me only about 30 or so and then Naruto took me the most with the remainder.

As far as Sasori goes I have no idea how that one works but I will trust what mods tell me. Even if each set has one or two ninja that are locked behind a preset requirement that seems just fine to me. Gives the top paying players a bit of exclusivity for now, which is fair.