The nine shadows are the Scabbards
Momo and Yamato should eventually get involved in some respect with the fight with Kaido, but the five already on the roof top will be the main fighters against Kaido, no one else.
I’ve seen nothing to suggest that Toki or Oden were capable of such specific prophetic skill to saw “yes, there will be exactly nine people who will fight and defeat Kaido.” For one thing, are we just ignoring the nine scabbards who each at least landed hits on him? Far as i see it, we’re already up to 14 participants.
Toki wanted to tell people the nine scabbards would come back to give them hope and separately Oden was aware through Sharly’s prophecies and whatever was discovered in Laugh Tale that there would be a massive war in twenty years where people strong enough to beat Kaido would appear.