An outcome is a goal a user can finish when the project ships. Most projects need one to three of them.
Why so few
A project's outcomes say what "done" means. When there are many, nobody can keep them all in mind: not the product lead, not the builder, and not the coding agent.
A short list also keeps the project small. See Split big work into small projects.
When a project has more than 4
Look at each outcome, and ask two questions.
Is it really a separate goal? If users could get value from it on its own, it's probably a separate project. Split the project.
Is it really a rule about a goal? For example, "The new time must be at least 24 hours away" isn't a goal. It's a rule about the goal "Move a visit myself." Move rules like this to the project's Requirements.
How Grove does this
When Grove writes a project's outcomes with you, it aims for one to three. If you add more than four, Grove suggests a split, or suggests moving some lines to the requirements.
This is a guide, not a hard limit. Grove won't stop you from adding more.