You may put as many features into a single module as you want.
However, the more complex your module is, the harder it will be for you to keep it compatible with newer X-Cart versions.
When adapting a module having a lot of features mixed in to a new X-Cart version, fixing one thing may break another one.
If you think the change that you are working on can be useful for other users/stores, it makes sense to wrap it up into its own module. You can even submit the module to
our marketplace later.