I don't think a soul mate is unique. There are probably dozens of soul mates out there but because they are so few and far between, you only meet one or two in your entire life. Following this, I don't believe there is only a singular "The Right One".
I do think that a soulmate is unique to every individual. But I also believe that you might not live happily ever after with your soulmate. A soulmate, I suppose, is someone whom you can share your deepest thoughts and feelings with, who understands you even better than you understand yourself, and vice versa. A soulmate feeds your mind, your soul and spirit through modes that works for you. A soulmate is able to provide something deeper than friendship and companionship that no one else can.
Just like you might not be in a lifelong relationship with the person you love the most, you feel the most for, the one most perfect for you, so it is with being with your soulmate. It's a bonus, if you are able to find a soulmate to be in a relationship with.
Simply put, the timing must be right.