Post count always 3 when using shortcode

I just installed and set up the Related Posts Thumbnails Plugin, and it works great when using the automatic insertion feature. But I want it placed in a different location than the default, so I tried using the shortcode [related-posts-thumbnails]. That still works, but it always displays only three posts even though I set the number higher. Is there a parameter that can be passed to the shortcode to specify the number of posts? Thanks!