The scrolls spent on each treasure, do not affect the other treasures. As far as I know, you can think of pulling ninjas like refinements. Each treasure has an average of 40-60 pulls, so my belief is that the cap is 70 pulls (free OR scrolls). So if you pull 1 on Tendo treasure, you have a chance of getting a designated rare, but if you don't then it becomes 1/70 or whatever the cap is. This continues until you get a Rare ninja. Each treasure is independent from each other and does not carry over. (This is all my opinion and understanding based on the information I've gathered.