The simple answer is to include as many routes, character arcs, and endings that you can do WELL and that your game calls for.
If you only have 2 character arcs and 2 endings, but they are both super well done, I'm going to like that a lot more that 4-5 arcs and endings that you only put token effort into.
One thing I love to see, but is incredibly hard to do well, is to have multiple character arcs and story branches that FEEL complete by themselves, but layer into each other as a greater whole, where players are really rewarded by going through the game multiple times by getting a deeper understanding of the characters and the story ... but if they don't, they don't feel like they had an incomplete experience by just playing through one character route and ending.
I forget the name of the game, but I played through one years ago that took a Rashomon approach of every route having an unreliable narrator. Only you didn't realize that was the case unless you played multiple routes. It was neat because you could piece together what REALLY happened only by comparing all the routes, because you never played the "true" events.
TLDR;
As many as can be done well, but Quality > Quantity.