I have a lot of thoughts on this, but the points of it is this:
Karma and I joke that we are truly soul mates in that- I keep coming back life after life b/c I am facinated with the way things change yet stay the same. He get's more and more irritated with society, but b/c we can't "live" apart, he drags himself along life after life.
Now we also believe there are souls we have encountered before and connected deeply with, that also come back through our lives-my wife, Karmas "brother" and several others for example.
Now to my understanding given the last time we talked- Karma does not recall any past life with g/f but there is obviously a strong connection.
I've had a couple exes that I was instantly drawn to but drifted apart from.
I think these were the soul mates of another life, always drawn together, but not really here to "do" anything for me or me them in this life.
In a round about way I do think you can have more than one. I personaly think it is something determined long before we cross onto this plane of existance. It's up to us to direct our lives through the lines the fates have created, in order to find those people. Sometimes they aren't on the right path for us to find them. Sometimes we cross 2 or more
at the same time.
Really all depends web of life created and what we do to help it along or tear it apart.