Images per page
This allows gallery images to display across several pages.
The following gallery layouts allow images to be placed across pages:
The Images per Page setting can be found in the Admin Panel > Image gallery (any one) > Advanced

Images per Page.

About pages in galleries
The number of pages in a gallery is the total number of gallery images divided by the Images per Page setting. For example, a gallery with 5 images, and 3 images per page, would have 2 pages. The first page would each have 3 images, and the last page would have 2 images.
Why is my Images per Page setting 999?
Some pre-configured layout suggestions will set this to 999 to help ensure that all images in a gallery appear on the same page.