Angelina Jolie says 51 million people have been displaced by the Syrian Crisis

I'm not sure where Angelina Jolie gets her figures but if we consider people in parts of Turkey, Syria, Iraq, Lebanon, Jordan etc. running for their lives first from Assad and now groups like ISIL then this might be where she is getting the 51 million figure.

51 million figure came from:

Angelina calls out 'failure of international leadership' on Syria

This is an awful lot of people. For example, in World war II it is thought that 50 to 75 million people (mostly civilians) died in this war around the world. And in the Cold War from 1945 until 1991 it is thought that 98 million people died from the Cold War during the almost 50 years it went on for what seemed like forever at the time in a sort of unending way.