As mentioned above, 10 scrolls guarantee a 3 star ninja, but it doesn't have to be one from the list. I still recommend doing it with 10 scrolls at once because it atleast guarantees a 3 star ninja.

Also as explained above: every time you dont get a rare the chance increases to draw one the next time and it keeps stacking untill you find one. From my experience I get one mentioned on the list about every 50 scrolls.