That depends. If you support several products, you might add a topic for each product. If you have one big product, you might add a topic for each major feature area or task. If you have different types of users (for example, end users and API developers), you might add a topic or topics for each type of user.
A General Discussion topic is a place for users to discuss issues that don't quite fit in the other topics. You could monitor this topic for emerging issues that might need their own topics.
To create your own topics, see Adding community discussion topics.