I believe that a soul mate is the person you know so well, that you can read their mind, and they can read yours. There is an unspoken communication right from the first meeting.
My 2nd husband was my soul mate and my best friend. He became ill and passed away in 2004, and I still feel as though I can communicate with him. I talk to him, and receive answers through other sources like TV ads, songs, words from other people (sometimes strangers), and yes, psychic mediums. I have actually felt him touch me, and I KNOW it was him. Nobody will ever convince me other wise.
When we first met, we would be in a room full of people, sitting or standing on opposite sides of the room and look at each other at the exact same moment, knowing what the other was thinking. That continued throughout our time together here on Earth. We would have died for each other. We would have gone through hell and back for each other.
I have loved before, and I have loved since, but loving or being in-love is not the same as the relationship with your soul mate. I look forward to the day I can "see" him again.
I'm not sure if its possible to have more than one soul mate in a lifetime. I am truly, truly in love with my 3rd, current and final husband, but we don't have that unspoken communication. We know each other very well, so we do communicate well...with words.