I am posting this to help other people in the future, as I just spent ages trying to work it out.
If you want to have multiple instances of this plugin with different quantities of images - eg. A big gallery page with 100 photos, plus smaller galleries of only 10 images - then use the shortcode "per_page"
I struggled with this because I put 100 in the "Number of photos to display" box and then couldn't get it to show less elsewhere.
[flickr-gallery mode="photoset" photoset="72157628722788633" items="11" per_page="10"]
10 = it will only show 10 images
Hope this helps somebody. :)