Skimming through the comments above, it seems like there’s no hard-and-fast rule for minimum endings. However, we should also keep in mind the type of audience you’re going to attract with your game.
In my experience, fantasy RPG fans tend to enjoy a variety of endings, with 3+ being the absolute floor (a good, a neutral, and a bad ending), but probably 5 is your real floor.
I think the 10 story endings you mentioned is actually right in that sweet spot of ‘a variety of endings, but not overwhelmingly so’.