There are many versions to the series. The main trilogy, 1, 2, and (soon) 3, then you have your extra/remakes/just stalling for time games that include 2.8. 2.5. X, and then some. Some others that add to the story element are Dream drop Distance, Birth by Sleep, Chain of Memories, among others.

The ones I have personally played are 1 and 2. I've only seen gameplay of the other games, or read the backstory to them.