Call of Duty Advanced Warfare has 45 Collectibles. There are 3 in each of the 15 missions. Collecting all the Intel will be required to earn the History in the Making (collect half the intel) and Ferrum Absconsum (collect all the Intel). In addition, you will need to collect all of them to fully upgrade your Exo to earn the Power Changes Everything trophy as they reward upgrade points towards that. You can get the collectibles through mission select if you missed one during your first playthrough. They will immediately save once you pick one up so feel free to save and exit a mission once you pick one up if you do it through chapter select.
The Intel come in the form of futuristic laptop computers that are easy to spot due to their brightly colored nature.