There is a total of 16 map stones in Middle-earth listed below
1 - Hobbiton
2 -Bree
3 -Weathertop
4 -Rivendell
5 -The Pass of Caradhras
6 -Lothlorien
7 -Amon Hen
8 -Emyn Muil
9 -The Dead Marshes
10 - The Black Gate
11 -Isengard
12 -Helm's Deep
13 -Edoras
14 -Minas Tirith
15 -Minas Morgul
16 -Cirith Ungol
Even after you activate your first one all will show on your map making them easy to track but you must still go to all of them and activate them one by one.
(If done on co-op only player 1 will get the achievement, if no achievement just go through the list again to confirm you have activated all of them even if they are on your map).