6 is Roku

Blackbeard’s bounty reads as San kokoro(ku) which means 3 Hearts/Souls.

Oda uses number puns a lot through out the story such 56 meaning gomu refering to the Gomu Gomu No Mi.

Blackbeard has 3 souls in one person. This essentially allows him to eat 3 devil fruits.

Jabra heard a rumor that an actual devil lives inside each fruit and transfers to a fruit user’s body upon consumption, and would fight with the devil inhabiting another fruit if they were to meet within the same body.

However Blackbeard has 3 Souls which can supress 3 devils.

Blackbeard currently has the strongest Paramecia and a special Logia so it would make sense for him to also have a Zoan, particularly a mythical Zoan.

We know that Luffy’s Devil Fruit is a special Mythical Zoan that was erased from history. Blackbeard might go after Luffy’s Devil Fruit because of this.

Luffy wants to be the freest man on the sea. For him to be the freest he must be able to swim.

Toki and Buggy both get really ill before Laugh Tale indicating that Devil Fruit users cannot make it to Laugh Tale.

Luffy will lose his Devil Fruit allowing him to go to Laugh Tale.

As Kaido said, Haki triumphs Devil Fruits and this will be displayed by Luffy in his final battle with Blackbeard.