There are 200 total Animus fragments all over the game world.North Atlantic
Most fragments require climbing, tree walking, leaps of faith, and/or jumping in place (
in any direction). Some fragments in North Atlantic require you to destroy icebergs to collect them with your ship.
The best way to find them is to synchronize any viewpoints or take over any forts or gang headquarters. Then the map will reveal all collectibles in the area. Simply set your map marker on the fragment you want. Then run/climb/etc to it. It's easy to determine the elevation of your target while your map marker is locked on it.
Here's a video showing all 200 fragments.
New York (1-41)
River Valley (42-123)
North Atlantic (124-200)