You can select them all and download them to your PC. It took me a while to figure out how to download all the images from the iCloud to my PC (Apple should make this more intuitive and easier.) but if you go to, select Photos, you should see ALL of the photos from your last backup. Just turning it off, however, doesn't make the photos all end up on your phone (maybe it would if you have room for them all, which I didn't).
If it's enabled, you're storing originals in iCloud. You can check if you have this enabled by going to: Settings > Photos & Camera > Optimize iPhone Storage. As it turns out, apparently the Optimize Photos setting was turned on which means the full resolution images aren't stored on your phone, which I think is what accounts for the missing photos. ChrisJ4203 had asked about iCloud and I'd mistakenly said I wasn't using it for anything other than backups. I actually had multiple DCIM folders in windows explorer, it just didn't show all the photos. So the cameras/scanners didn't work for me either.
