It is quite complication so please go to: www.kktanhp.com/twin_souls.htm
I have an opposite opinion to that described in the website link above. I believe that one can have many twin souls and only one soul mate. Twin souls are family members or close friends who are very much like you, ie they share interests, opinions, and react to things much in the same way you do. You "get" each other.
However, you only have one soul mate. This person may share many qualities with you, but isn't exactly the same. Their qualities fit with yours so that together, you complete one another. Because you do not share all of the same opinions, you can grow and improve one another, rather than agreeing all the time.
Think of it in these terms: You can have many of the same left shoe, but the pair is never complete until you've found its mate!