How many branches should you have on one storylet? You need at least one (and you need one that’s always playable on any Sometimes storylet). But how many should you have?
To a certain extent, it’s a matter of taste. If you’re doing a chain of ten branches, you could have ten storylets, each with a single branch. Or you could have a single branch with ten storylets, each set up to only be visible at the right stage in the story.
One branch per storylet has the advantage of more text available. You have more root text, so you can lay out each situation a bit more before the player plays the action. But it’s a bit harder to keep track of, and you do have to write out all that extra text whether you need it or not. The opposite is true for multiple branches. It’s easier to keep track of because it’s all in one place. But your root text is shared across all the branches, which might not work so well for you.
The best thing to do is to experiment with both and see which works best for you, and in which situations.