I think, in total, there are 8 treasure chests to find.
To open the chests you have to buy the lockpick from the Merchant Moraekah first. You will find her at the end of the SideQuest "Skill of the Player - Dice with the Dead" in the Hidden City (There is a Boy near the wooden bridge who claims that a dead guy stole his dices - Speak with him to start the quest).
You will find the treasure Chests in the following locations:
1 in Cozumel (in the final room where Lara took the dagger)
1 in the Peruvian Jungle (near the first campsite, climb down the cliff)
2 in Kuwaq Yaku (one in an underwater cave below the southern monolith and one in the underwaterare where you first encountered the piranhas)
3 in the Hidden City (two in rebel part of the city and one more in the cultist part)
1 in San Juan (in a Cave by the lake, south of the mission of San Juan)
Thank you WKFJonesy for providing the map: https://camzillasmom.com/shadow-of-the-tomb-raider-2018/inde...