There are 38 hidden templar chests in the game and these can be found in all three of the main areas: Boston, New York and the Frontier. Most of these will also force you to undertake the game’s annoying (and thankfully under-utilised)lock picking mini-game.
Note that these will appear on your map after using view points, if you come into close proximity to one or by purchasing a map with all their locations from one of the general stores.
Here are the general locations of each chest, based upon regions:
Boston (13 Chests)
New York (12 Chests)
The Frontier (13 chests)