Evidence are items pertaining to the myth.
For example if the Myth is "Golden batteries are a collectible in JJ2" a peice of evidence would be the battery. Thered be other stuff like "Tileset used" which would be a small icon of a tileset.
One or two pieces of evidence are enough to complete a myth, let's say the two peices of evidence are "Gold battery" and "Sirus tileset used".
Thats enough to confirm a myth that is obviously false.
Theres anout eight or so optional peices of evidence, the higher you get the more likely you'll bust the myth.
However if a myth is something like "The Speed shoes in JJ2 don't work" Its the opposite, and you're trying to confirm the myth. A lack of evidence will bust it.
Succesful completion of myths adds to some other kind of ranking, and the higher this other ranking is, the more NPCs will talk to you. "Failing" a myth (as in, just getting the bare minimum evidence) would subtract from that ranking. The lower this other ranking is, the less NPCs will talk to you, and the ones that do will probably do it in a mocking tone giving you myths like "Norm turtle isn't an enemy in jj2".