Scrapbook Albums come with clear 12 x 12" plastic sleeves called Page Protectors into which you insert your completed page layout. The pages can be removed and reordered by adjusting the screws on the spine of the album. You can have your album holding more layouts by adding more page protectors.
Scrapbook patterned and bazzill plain paper is sold in 12" x 12" size, hence the size of these albums.

Refill Page Protectors for Scrapbook albums.

A popular smaller size Scrapbook Album is the 8" x 8" size.