Sometimes when they get saved, they are saved out of order. Also, they can wind up overwriting others if the name/number is the same.
I would suggest you connect the phone to a computer via USB cord and move the photos over to an empty folder on the computer. You can then more easily sort through them there. Once you are done, copy them back over the phone (if you wish to), but don't 'move' them back to the phone. As that would remove what is on the computer. Having it on the computer is a great way to save your photos should the phone die, be damaged, lost, stolen, the photos be deleted, etc.