If you plan to buy a used camera or you want to sell yours then be ready to answer this question: How many shots where made with this camera?
Why should i know image number
Well, each camera has a shutter mechanism capable to move extremely fast and as with everything mechanical, it may need to be serviced or replaced someday. Manufacturers usually guarantee between 100,000 to 400,000 shutter actuations. (Find instructions manual for details)
In each camera there is a built-in counter that keeps track of these actuations. However the shoots number are not displayed in any of the menus so you need to find it by yourself.
How to retrieve it
1. Set your camera to shoot RAW photos (.NEF files) and take a photo
2. Transfer the RAW file to your computer
3. Open it in Photoshop
4. Navigate to File > File Info… > Advanced tab
5. Twirl down the Schema folder and you’ll see the Image number indicator.