Change the number of entries depending on a theme setting
You could do this, but it will require that you know a bit about views exports.

You would have to create the view in code, by exporting it, this could be done with features or the views alone and use hook_default_views.

Once views read the view from code instead of the database you could insert a dynamic value for the number of items shown by the view, gotten from the theme setting. You would need to make sure that the value is formatted the way views expects it.

One important note. You can't make any changes to the way, as it will stop reading from the code and use the database instead. When this happens the dynamic value will be replace with a static one.