There are 30 landmarks in the game. You unlock a landmark when you drive or walk by the location of a landmark. None of them are missable and all landmarks are immediately saved after you discover it. I recommend that you discover the landmarks in free roam mode, which you can enter after completing a desk (except for patrol) from the main menu. The free roam is called "The streets of L.A.”
If you want, you can use the map and the tracker from Rockstar's social-club by connecting your xbox one account.
https://socialclub.rockstargames.com/games/lan/checklistIf you go to the social club, look under "landmarks" to see how many and which landmarks you have already discovered and not discovered. The map from L.A. shows you all the locations, but for some locations, the map is really inaccurate. That is why I have added my video for better clarification.
A note: My video is a map video. You only see the locations on the map and how to unlock a landmark.
Thanks to GameGod03 for the correct text.