As the previous comments have mentioned look everywhere, in areas that you might not think to go. It is time consuming but you'll get all of then in the end.
It would help if this game gave you Full Stats in the menu when playing the game but it doesnt. Though once you've played through an entire chapter, Restart It, and now check in your phone for your briefing etc. Press right trigger until you come to intel collected and it will show now how many you have collected.
If there's 1 or 2 missing thats how many who've to find in that chapter. If all are collected in a full chapter and you still dont have it, then restart & got to the next chapter and check in that ones intel.
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