That is not right either. They can not send you a photo at 72dpi. DPI is printer resolution, not photo res. Photo res is ppi. A Photo 6000x4000 is 24,000,000/24 meg. you can print the photo at 1 dpi or 300 dpi and the res of 3000x4000 does not change. What changes is the printed image res. It will look different printed at different DPI and viewing at different distances.

As I said, go back up and read Gene's post. I believe this is the site he listed. If not thisa is good reading.

http://www.photokaboom.com/photography/learn/printing/resolution/1_which_resolution_print_size_viewing_distance.htm

Gene's exactly right. It is a combination of resolution, size and viewing distance. There are entire billboards that have photos taken with an iPhone. That is misleading because one views billboards from a longer distance away and cannot see the pixelization. It is no different than looking at mosaics in churches in Europe. They look like fine paintings at a distance, but up close one can easily see the smaller bits of tile. The same is true with photos. Or go up at 2 inches and look at your tv.

This is kinda why some of the super high resolution cameras really are an overkill for many people.

Your D500 images will, given that you take sharp well-done images, be able to be enlarged beyond the 1:1 ratio of the camera. You can do this in a number of software programs, including LrC and PS if you have the Adobe plan. If you go to an outside printer, like Bay Photo, they can assist you with specific information. Normally, with an outside printer, you would simply load the largest file you have. In actuality, often it does not take a really large file to create a quality print in larger sizes, depending on the image quality and the print size.