There are 2 carousels - one for Storyboard homepage and one for Media Room Homepage. The setup page also contain 2 options tabs (Homepage/Media Room Homepage).

There are 3 positions in each carousels which are automatically filled by latest Stories in case of Storyboard and by News in case of Media Room. You can set up what content type is used for automatic filling. See Automatic post type list of checkboxes.

If you select 1 article from Left list to Right list, you will overwrite one position in carousel. If you select 2 articles, you will fill 2 positions in carousel. Third position is still filled by latest article. So, you can stick up to 3 article in the carousel despite there are 3 more newest articles than your sticked ones.