There are 24 duels in the ZEXAL campaign to beat. DLC duels do not count and Recerse Duels are not required. You can use either the 'story' deck or your own constructed deck.
For those of you who want to burn through these duels as fast as possible, here's an Exodia deck recipe (credit to Bewch on Steam):
Monsters x 11
-Left Arm of the Forbidden One
-Left Leg of the Forbidden One
-Right Arm of the Forbidden One
-Right Leg of the Forbidden One
-Exodia, The Forbidden One
-Broww, Huntsman of Dark World x 3
-Royal Magical Library x 3
Spells x 29
-Broken Bamboo Sword x 3
-Golden Bamboo Sword x 3
-Dark World Dealings x 3
-Graceful Charity x 3
-One Day of Peace x 3
-Pot of Duality x 3
-Pot of Greed x 3
-Spell Power Grasp x 2
-Toon Table of Contents x 2
-Toon World x 1
-Upstart Goblin x 3