There are 30 Intel Locations (Collectibles) in Call of Duty 4 Modern Warfare Remastered. Finding all unlocks the "Eyes and Ears" trophy / achievement and several cheats (such as infinite ammo).
None of them are missable. You can get them via mission select at any time. In mission select you can see how many you have found in each mission. Your total amount of intel can be seen in the "Intel Cheats" section of the main menu. While on a mission you can also see the number of available intel by pausing the game.
All intel pieces are saved instantly when picking them up and you can quit out to the main menu right away without losing them. You do not have to pick them up again if you die.
Finding half the intel earns you the "Look Sharp" trophy or achievement.