According to the wiki, there are only five "Desmond's Journey" memories (warning: spoilers on that page). However, if you look around the area where the five memories are located, there are two more gates present.
The gate on the far left is the fifth memory, and the other two are currently empty. I assume they aren't being put there for no reason, so what's their purpose? Is it for DLC in the future, or are they already used for something?