Thanks Dr. W. Regarding your query, You can select any post category and number of post in carousel shortcode. It will pick the latest posts from that category.
In case you have selected two different categories, say X posts from Category 1 and Y posts from Category 2. Then it will pick X latest posts from Category 1 and Y latest posts from Category2, merge all posts into single list and show posts randomly from this list.
So if you will select only one category then it will pick latest posts from that category only.
Hope this will clear your doubt.