Despite all difficulties trying to control Altair with the touch screen analog stick, there are 13 chapter/memories you need to complete for this achievement. There are some sub chapters/memories for each. To plan your game out here are the following chapters/memories with each sub-chapter:
1-1, 1-2, 1-3, 1-4 after you complete the last one, you will be awarded with
2-1, 2-2, 2-3 after you complete the last one, you will be awarded with
3-1, 3-2, 3-3 after you complete the last one, you will be awarded with
4-1, 4-2 after completing the last memory here, you will unlock
5-1, 5-2
6-1, 6-2, 6-3
7-1, 7-2, 7-3 after completing 7-3 you will be awarded with
8-1, 8-2, 8-3, 8-4
9-1, 9-2, 9-3, 9-4 after these four sub-chapters you will unlock
10-1
11-1
12-1, 12-2 this will be your second to last achievement regarding the story line
13-1 and finally your game should be completed now with