Since I had to struggle a bit to find all of the viewpoints, I went ahead and took screenshots and marked them. The two that most people seem to miss most are in New Orleans and mostly obscured by other icons; 1 on the roof of your Assassin's base and one in the southeast near the docks.
Also, as with many Assassin's Creed games when you first reach an area some of the viewpoints won't be accessible during that memory
New Orleans - 11 ViewpointsThe Bayou - 6 ViewpointsChichen Itza - 2 Viewpoints