He's a rare in kage treasure, which means on average it takes about 60 seal scrolls to pull a rare. Since there are 7 other rares besides killer bee in the kage treasure, this means you will have a 1/8 chance of pulling him. You might get lucky getting him in your 1st rare pull or your 10th rare pull since you can pull the same rare you got again.

This is why I recommend you just to go after han and roshi as a water main since you know how much seal scrolls on average you need to spend. Killer bee has appeared in the past but with new ninjas being introduced, it is more likely he won't appear a lot.