The concept of a soulmate, and all of its inherent difficulties, makes a lot more sense in belief systems which embrace reincarnation, especially those which also hold to some concept of karma.
An individual soul comes into this world more than once, and in different bodies. That soul interacts with others over its many lives. Conflicts occur, as do more amicable interactions. People and other incarnate souls spend quite a bit of their lives settling various kinds of issues with others. The issues -- good and bad -- between two souls are not always resolved before one or the other disincarnates, however.
Under the Hindu concept of karma, how you conduct yourself in this life determines what you come back as in the next. Most pagans believe that you reincarnate in order to resolve issues left over from the previous life, or to learn some sort of lesson, or to apply the lessons that you learned. With those two things in mind, let's reexamine the soulmate concept.
The purely romantic notion of the soulmate is someone you are destined to share your life with in all aspects. That is a rather narrow view of the whole concept. If two souls have unresolved issues from past lives, then it would follow that the forces of destiny would have them draw together in order to resolve those issues. Karma is ultimately about balancing the books spiritually -- you push on the universe in some way, and it pushes back. A corollary to that is that when the universe is pushed, it will naturally tend to shift back toward equilibrium. The interactions between souls push on the universe in a multitude of small ways, and only resolution of those issues brings things back into alignment -- and the tendency toward equilibrium will bring those souls back together, time and time again, until the issues are resolved.
So in a lifetime, you're likely to have more than one soulmate, in the broader sense of individuals whose karma is tied to yours in some way. (They're not necessarily going to be human.) You're not necessarily going to know it at the first meeting. Only when you realize that someone keeps coming back into your life, or enters it and doesn't leave (especially if you want them to leave), will it become clear. And it's not always a good, happy, romantic thing. You and this individual are coming together to resolve leftovers from past incarnations, or to teach each other necessary lessons. It's not always pretty.
And when you think about what kind of issues in your past lives could have brought you together in this one, it can be downright sobering. In the case of romantic soulmates, two people are fated to love because they didn't love each other enough in previous lives. Was it because they were great lovers for whom a single lifetime of love was not enough...or was it because they did each other some great wrong, and are brought together this time to learn love for each other? Or, since love and hate are not opposites, but are merely two aspects of passion, was it complete indifference that caused it?
So don't go looking for a soulmate. They'll be brought to you, or you to them. There will be more than one, unless you can go through a whole series of lifetimes without leaving unresolved issues in place with more than one soul. And the interaction between the two of you can run the whole spectrum of emotion.