Looks like I got all the chests in the game. How can I make sure? I got all the Animus Fragments already. I also got the Templar Armor and Mayan Armor.
It will show up in your progress tracker (available in the Options menu) under either side activities or as a check within the specific locations.
There are 'social chests' which appear randomly, and there seems to be an indefinite number of these, however these don't count towards your completion percentage. (Other than sharing some in the Abstergo challenges).