There are quite a few quests. Doing the math based on 100% and the percentage I was obtaining after each mission, there are around 250. You have your Requests, Legends, Gaiden, Crossover, and chapter missions. Legends unlock as you find them. Gaiden unlocks upon completion of all chapters and they are all high difficulty. There are 52 of them, and they unlock as you beat them. Crossover will appear by the request board, initiated by unplayable characters from other games. You will find them as you go through the game easily. On Chapter 3 you will see the gatekeeper in the front of the town. He has three missions for you as well. I did all of these without trying or keeping track, as the game flags your progress. You do not have to play the DLC or High Difficulty based missions you have done previously. You will want to play the ones listed later to unlock all characters.