I would like to show more than 20 photos in a gallery
This is not currently possible as Flickr RSS feeds only return 20 photos.
If there is some demand I could process multiple tags which concatenate the results. So rather than tag=”tag1,tag2″ returning the 20 most recent photos with either tag1 or tag2 I could form a collection of the 20 most recent photos with tag1 and the 20 most recent photos with tag2. In this way larger collections could be built.
This approach still relies on not tagging more than 20 photos with the same tag so it is not ideal.
In August 2010 Slickr Flickr 1.14 was released which allows up to 50 photos if you specify your Flickr API key.
In December 2010 Slickr Flickr Pro was released which allows an unlimited number of tagged photos, group pool photos and favorites