There are a couple strategies for creating a list that includes an element to expose more list options.
One option is to create a list containing a
<summary> element. The primary list would contain however many options you want to show and the
<summary> element would contain a "Show more" disclosure that exposes the rest of the list when activated.
Another option is create a list that includes a "Load More" button, which will dynamically load the rest of the list options, instead of using a
<summary> disclosure. Then, use
aria-posinset to represent the overall list size as options are dynamically loaded when the "Load More" button is pressed.