Maximum displayed image width and main column width have nothing to do with your question.
a) When you insert a gallery into a post or a static page, you can set the number of columns you prefer if you select the "Thumbnail Grid" type of gallery. If you select one of the other types, you cannot change the default number of columns.
b) You haven't published any posts, and you've set a static page as your front. The 5 columns of circular images you can see on the default front page of Spun aren't a gallery inserted into a static page, they are the main posts page. Each image represents a post and links to that post. Check the demo and click on any image: