They only need to be as deep as your shoes are long. I used the 12" shelves when I put mine in.

Taking note of the width and height of each opening, ask yourself:

Will the pair -- both shoes -- fit in the 18" deep slot? If not, or if you just don't want to reach into a dark nook (not fond of spiders; difficult to clean, etc.,) you might consider a vertical drawer for your shoes -- preferably with flat shelves with adjustable shelf heights.

https://cdn.homedit.com/wp-content/uploads/2014/02/bed-with-storage-for-shoes.jpg

https://i.pinimg.com/736x/50/60/ed/5060eda1c592f479567a2b103ff4edf7--el-interior-interior-ideas.jpg

http://www.csomerlotdesign.com/cdn/image/innovative-pull-out-shoe-storage-pull-out-shoe-storage-shoes-design.jpg

12 to 18 inches depending on your shelving.

This product, glides out like a drawer stores "up to nine pair of women's shoes (flat or heeled) and up to five pair of men's."

mine are 16" and sometimes I can double stack. 12" is too short (unless you have size 6 feet, then it's fine). I'm a size 10 and the 16" is perfect for my shoes and boots.