How to Change the Number of Image Columns in a Gallery

1. Click Photo Gallery from the menu on the left of your WordPress dashboard. 
2. From the list under Photo Gallery, click Options, then click the Gallery Defaults tab.
3. From the window that follows, find the Number of Image Columns setting, and click the box to set the number of image columns you want to display.
4. Click the Save button at the top right to save your settings. This will apply your selected option to every gallery that uses default options.

In order to apply this setting to a single specific gallery click the shortcode of the published gallery in your editor, uncheck the box next to Use Default Options, then follow step 3 and click Update.