you get Ay or Sasuke susano'o at 900 draws (the free draws you do matter for it, that's why people say 660-720 or other lower numbers, it's simply because they drew 720 times with seal scrolls and 180 times as free draws so they reached 900 that way. People that say over 900 i simply think that didn't remember correctly how many draws they did).
About itachi you get 1-2 fragments for sure every 60 draws you do it's a fixed drop.