That's not much to go on.
What's different about the ones you can see and the ones you can't? Are they in different folders, and if so have you told Photos not to look in some folders (or do the folders have a hidden file called ".nomedia" in them which stops it looking)? Are the ones you can't see downloaded rather than taken with the camera, or things that have been deleted from the device, or...? Has it always been like this or have some images suddenly disappeared? Or gradually disappeared? Are the missing images in the internal storage or on a microSD card, and if they are on a card how long have you had it and where did you buy it from (because a lot of cheap cards are "fakes", which claim to have more space than they really have, and data stored on those are likely to be overwritten)? Can other gallery apps see them? Can you see them with your file browser?
Basically there's more than one possible reason why not all of your images might show, so the more information you can give the better the odds that someone can figure it out. Some of the questions above are just trying to narrow down the possibilities, others are asking about specific problems that might occur.